Mobility becomes important only if angle of attack and formations become important.  If rolling up a flank isn't a meaningful maneuver, then mobility is relatively unimportant.  It's hard to reflect mobility properly in a turn based game.  One possible way is to allow attacks to occur at any point during movement - that would allow, say, horse archers to wheel close, shoot, then retreat.  If automatic counterattacks are implemented, then there should be an advantage to attacking first - that is, attacks should degrade unit effectiveness.

Historically, units tended to lose because morale broke, not because everyone died.  Veteran units tended to have better morale and so to hold longer, even though they might take more casualties.  And casualties are not the same as deaths, particularly where magical healing is available.  Adding morale would make things a lot more complicated, though.

When the unit is boiled down to Attack, Defense, Hit Points and speed, experience, by definition, has to improve one or all of those.  If you want experience to have other effects, the unit has to have other dimensions.  One dimension that hasn't been talked about but is likely important in the game is special abilities.  Experience may either grant special abilities or help special abilities to work better.  For example, all pikemen may be able to form a square to defend against cavalry, but maybe it takes a turn or two to implement the formation - veterans could do it faster.  Or maybe only veterans can do it at all. 

Kohan did a lot with formations but also used a real-time combat approach.  Still, the formations were interesting and may be worth looking at.

Experience may also have an "aura" type effect - strengthening the backbone of nearby units (i.e., supplying combat bonuses).

Incidentally, there's nothing wrong with experience just increasing attack, defense, hit points and speed.  I'd suggest allowing the player to pick where the experience goes rather than just increasing everything uniformly - that would allow the player to emphasize unit specialties and give more of a sense of growing the unit under the player's control.

Another idea is to add personalities to veteran units.  They'll pick up a veteran corporal or a veteran sergeant - a named individual that grants bonuses to the unit.  Could even let the exceptional veterans be transferred between units - give a green new unit a veteran sergeant to show them the ropes.  Might have a special ability of increasing the rate at which experience is gained.  A veteran medic might allow the unit to recover HP each turn.

I like the idea of making terrain types matter as a battle setting. Hilly terrain could have better archer points that add a unit of better range and visiblity. You might also hide some units behind hills so the enemy doesn't know their exact position. Heavy woods would could reduce cavalry speed to footmen speed.

Disagree on the environmental damage types though, it doesn't make sense. If I'm wearing snow gear then I'm supposed to take more damage from a mage casting a giant snowball? If anything it should be the opposite. I could see however a need for different gear types or your units take damage. Wearing furs instead of metal armor in arctic regions or else suffering cold damage every round (as well as other effects potentially). Likewise deserts are too hot for heavy armor, see the history of the Crusades and how that failed them.

This models the history of warfare in which many civilizations mastered only the art of warfare in their region and their military advances were halted by climate they weren't suited to do battle in.

Rishkith, it sounds like you agree with the environmental damage but disagree with the specific example. Am I right?

I'm somewhat tired of swords. Or, to be more specific, of the way that most games tend to approach weaponry. Swords are always the default, usually the best, and when they even remember that other types of weapons exist (you know, the stuff that was actually used far more than swords throughout history) then they make it into a lame rock-paper-scissors type of affair. And that's boring and frustrating to me.

I want to field an army equipped entirely with spears - personally, I'd prefer to see spears be the default weapon type instead of swords, but that's just me. I also want to field armies that use axes, or maces, or whatever else I can think of. I'd like to see a whole variety of weapon types for equipping my troops, with different benefits to each.

And they all need to be valid choices. I don't want to end up getting punished by the game because I picked one option and the AI soverign picked another. I don't want to play rock paper scissors, I want to build up my kingdom with what I think is cool.

For me this is the issue with Mongols as a viable army. Historically, the Mongols were a pure cavalry force, highly disciplined, highly mobile, with superior tactics and logistics. In a typical strategy game, it doesn't work that way - cavalry beats archers, but spearmen beat cavalry, and archers beat swordsmen, so every sensible army is going to consist of equal parts all three. That doesn't make for a very diverse tactical environment where all armies are fielding the same kinds of units.

If you say 'Spears beat Cavalry' then no, Mongols are never going to be a viable army because when you make an all-cavalry force then the other person will build an all-spear force and trounce you.

This.  Start with master of magic's combat system.

Experience can be handled like in fall from heaven (as i remember MoM didn't do much besides increase stats at level up) - discrete promotions units get at each level. 

Stay away from making tacitcal combat as involved as, say, the total war series.  Master of magic really is a gold standard here in terms of how much complexity you should have.  Fights were fast, but could be very involved and interesting.

About experience : add things like stamina and morale. Then an experienced warrior won't lost morale as easily as an inexperienced one. Same thing for stamina. For instance an experienced warrior can redirect damage done to the stamina instead of the health, making them more able to survive in combat.

In martial art, the first thing you need to learn is how to survive a battle, how to avoid things.

One more thing : experienced warriors could get better results for damage in the range they're allowed to.

For instance if a weapon does 1 to 4 damage with same chances to get a 1 or a 2 or a 3 or a 4, then an experienced warrior would get a better chance to get a 4 than a 1. (that system is used in Sacred 2)

About mobility : you use some of your movement point sto initiate an attack. If you win in less than two turns then you will get back some movement points to simulate some kind of blitzkrieg.

EDIT : One thing I hope ... don't make a system too simple. Battles will be a great part of the game. I just hated the battles of galciv2 (in fact i stopped playing it because of the system). Simple, yes, but not simplistic, pleeeeeaaaaaaase.

Another thought, which ties in with the casualties vs deaths comments above.  I'd like to see an after-battle phase where the winner can optionally pursue the beaten army (perhaps only in battles of a certain size?).  It's something that occurs in some board games, and nicely conveys an advantage to an agile army.

So, when one side decides to withdraw or is routed, there is some form of calculation based on speeds of the armies, time of day, terrain etc.  Possible outcomes involve being wiped out, escaping cleanly, rear guard actions and so on.  Good results mean a lot of casualties can be recovered, bad results mean, well, they don't.

Each player could have an option to withdraw at any stage during a battle, but they run the gauntlet of the enemy's pursuit.  This way, player's can commit their sovereign to a battle, knowing that they still have a chance to withdraw if things don't go well.

I don't have much time right now, but before I explain it in detail later I want to point out the major factors early:

1. Agreeing to most of my predecessors Morale should be a most basic value, that should change throughout a lot of the battle by many factors (experience of the unit, win/loss, difference in numbers, direct hits, a lot of magic, creatures that produce fear/terror, there is a lot of space.) The effects of Moral could be different, but should result in a gradual loss of overall fighting capability (including mobility) maybe up to the point of fleeing. You also wished to stronger include experience, well here is the best spot to do so. Be it a factor to decreas morale loss, more morale to begin with, faster morale regain or any combination of it, experience should affect it. (Because that is exactly where in our real world experience had the biggest effect)

2. Rock, Paper, Scissors should NOT be used in a artificial way. The game design should always be, that the classical balance from our real world (examples: cavalry beats archers most cases), RESULTS from the abilities and attributes of the units. I give classical examples. Cavalry shouldn't get a bonus against archers because they are cavalry against archers. They should be good at it, because they are fast and can outflank enemy positions, and because archers tend to be vulnerable to charges because of their lack of shields and formation. Another examples is spears against cavalry. Spears shouldnt just do bonus damage to cavalry. They tend to work against cavalry because they were long and cavalry tends to impale itself, so maybe give them something like a first strike ability and/or a damage bonus against a charging unit, that scales with that unit's charge bonus. I hope you understand the idea.

3. The battle needs to go far beyond the basic attributes, that are currently implemented now. (This is my personal opinion). As I understand you devs want to give the players a feeling that their "Zhaak'Ye Archers" or their their "Hounds of Karesh", or other units and their combinations that they come up with, feel special to them. You CANNOT reach that with a simple attribute system as it is implemented now. If you design a unit that simply has the best of everything (maybe except mobility) and anyone can do without spending a single thought on it, it means nothing in the end.

In order to fix that, you either need to complexify the base system a lot. You do that either by complexing the base system: Damage Types likes Blunt, Piercing, Slashing, whatever, just an example, the corresponding armor types too then.Chainmail is nice against slashing and ineffective to blunt and piercing, to give an example. Special abilities tied to weapons (first strike, charge value, one-time usables etc).

Or you promise us a very complex and distinctive magic enchanting system for units or units equipment, better both. I will just give one example and that is the well known Master of Magic. The combinations of enchantment-spells on units (and their running costs, so you had a tradeoff), are up to today unmatched in their complexity and fun to come up with.

To my personal liking I would want to see both, a complex equip-based base system with damage types and enchanting options (with a tradeoff), for equip and units respectively.

4. Also agreeing to a predecessor without many words: You at LEAST to differentiate between MAGIC and PHYSICAL damage types. Personally I think an element-system should be included (Its called Elemental after all isnt it )

Alright, that's it for now, I hope that my argumentation is plausible to you,

Have a nice day,

Katerchen

A) A simple Zone of Control (ZOC) mechanism will be definitely fun.   Your enemy cannot bypass your surrounding 1 hex ZOC unless their SPEED is way higher than yours.  However, if you only have a few units in your stack, your enemy can bypass your ZOC if they are way more numerous (e.g. they have total HP of 10,000 vs 500HP of yours). 
 
The Mongols will have a very high SPEED so they usually can bypass opponent ZOC.   The Mongols also has high MOVE that allows them travel several hexes per combat turn.   Therefore, Mongols can ‘outflank their infantry-heavy opponents’ as asked in OP.

B ) Free opportunity strike (Optional, it depends).  When one party decided to leave an opponent’s ZOC, the opponent is granted a free melee attack, if the ‘total HP’ difference is not too big.

C) Flanking.  Enemy managed to get to your back has ATT & DAM bonus.   I want it simple here, just the back, but not the side.

D) Battle Tactics.  Some specific units can perform special ability that push/pull their opponent on the tactical combat map.  For example, if your dragon can pull your enemy one hex closer, so they become within your archer’s attack range.

E) Unit formation.  If units can occupy more than 1 hex, unit formation can be fun, when there is flanking/battle tactics and ZOC dynamics.   Otherwise if all unit only occupy 1 hex, I don’t see unit formation is any much more fun than the boring Blunt weapons vs. Cutting weapons mechanism.

F) Cover %.  Each unit has a Cover % that will be added to the terrain’s inherent Cover %.  This % is used for projectiles attacks like arrow, canon.   For example, a dense jungle has a natural 100% cover, making any projectile useless when someone hide there.    Another application of Cover%; breath weapon check against Cover%, but it will naturally cut the effective % to half.

G) Luck %.  It is the resistance against any mental/status attack like Confusion, Blindness, illusion attack etc.  When the troop get more experienced, its Luck% goes up with it.   Also, lucky units has a greater chance granting them a free extra attack.   These should be one of the way experience be used more than as simply a modifier.

Ideally the calculation of damage is done by using PART OF the “Material physics” mechanism I’ve proposed.   This mechanism should allow extreme flexibility on how different type of damage can be dealt to any material; any modder should be happy with such.  Denryu, this Material physics idea will work well to your question.

If such a flexible system is too complex to program, I will then vote for the Magic Resistance system many people has already mentioned.

The problem with rock-paper-scissors isn't just that it's boring to me - it relies on the sort of structural framework that won't actually exist in Elemental because you're creating your own army from the ground up. As Frogboy has said, there is no 'knight' unit, so you can't balance units with the assumption that 'knights' work this way and 'footmen' work that way, and there are established relationships between pre-defined forces.

Take Cavalry, for instance. Cavalry is just 'a soldier on a mount' by definition. In Elemental, you can put any soldier you want on a mount, give them whatever weapons and armor you like. It could be a soldier with a sword, an axe, a spear, a bow. They could be using a shield or not. They could be armored or not. They could be poorly trained, or elite units. They could have various enhancements on top of that.

It's the same with every other unit. Archers beat Spearmen? Really? What about if I'm outfitting my troops as roman legions, with long spears, square shields, and armor. A turtle/hedgehog formation is hardly going to fall apart from an arrow barrage. What about skirmishers with short throwing spears or javelins, and light shields? The Archers will have better range, but they'll have better defense against arrows.

There's too many variables for just 'this unit beats that unit'. There is no 'this unit' or 'that unit' by default, there's only the units you yourself create. If you balance it on the assumption that spears beat cavalry beat archers, then where would you fit in any of the other weapon types? It's overly simplistic.

Let it be determined by the individual statistics of the units, and simply give the equipment types their own traits. A mount lets them move faster. Heavy armor raises defense but slows the unit down. Shields block attacks but only allow a one-handed weapon. Polearms give them better reach, maybe Axes damage shields and Maces penetrate armor. Bows or throwing weapons give a ranged attack.

Don't slip into the trap of trying to ape the simplistic gameplay mechanics of other games. You can't base your assumptions around rock-paper-scissors because there's going to be a virtually unlimited number of unit combinations. In real life, it was never just 'this unit beats that unit'. It was that this particular unit with its equipment, training, and tactics, has an advantage against some other kinds of equipment, training, and tactics.

I'd like to see combat kept fairly simple. If you're planning on a MoM style tactical map for combat, you can create enough variation for the attacks themselves through attack/defence/hitpoints and a handful of special abilities/civ 4 style promotions. King's Bounty is a recent game that did simple combat pretty well. King's Bounty had attack types, but if there's other parts to the game I'd rather each part was reasonably easy to pick up.

Complicated only equals fun for people with lots of spare time, so I'd rather you didn't cater exclusively to them.

If you do want to cater for them, a good (or generous, either works) autocombat feature is a must so more casual players can play without being steamrolled by AIs that understand the hundred page attack matrix.

Physical attack - I like blunt, piercing et all

Magic attack - basic elements, death, light, whatever fits to the game story

HP/Armor - Maybe damage mitigation as in Demigod for armor? Or just HP.

Important things about combat: Keep the numbers of stats low. That means that HP should go up to 20 or something like that. Same for attack, defense, armor etc. Why? If you get something that gives you  +1 boost, going from 5 to 6 makes it feel like you got something. Going from 53 to 59 just doesn't have that feeling.

Always display what is happening - damage dealt, attacks avoided, poison damage, etc. Demigod does a good job there. It is very important that player sees what is going on and that you can see all those criticals, insane damage and combos you so carefully produced by powerbuilding your unit.

Height & position (flank, back) should give you an advantage (if you ever played D&D, +2 when in doubt:)

D&D also has a concept of attack of opportunity which are nice but probably too complex for Elemental.

Destructible city walls, buildings and defenses (similar to Age of wonders)

Magic system - I always liked Magic the gathering system and MoM used a lot of it. I'd love to have very complex magic system for tactical battles. Global map enchantments (rules from main map), battle enchantments and unit enchantments. Instants (like fireball) are probably already planned and interrupts are probably too complex to handle.

But getting into battle with just a couple of units against numerically superior foe and then mind-controlling all enemies and making them fight among themselves should be fun. You know, not the "army with highest DPS wins"...

And for last suggestion, if you are feeling bold, don't use HP at all. Have a state machine define health of the unit.

Something like:

State Healthy -can do anything

Attack  with power X -> f(X) chances to Set State to wounded

State Wounded -> moves at 50% speed, can only perform some actions

Attack and in state wounded - chance of becoming Dead or Fatally wounded ...

You could even implement some internal HP counting to increase chances for specific states. And of course, each unit /unit type / unit body would have unique state machine.

Why do that? This would give units completely unique feel. For example, you could make light attacks just make dragon angry - increasing attack or speed. It would also remove some of the determinism from the game - if you have been playing the game like MoM or AoW for long enough you know when you are going to lose...

I'm happy you're planning on making height and cover factors in tactical combat, that is good news!

Although it's all been said before:

Doing away with different types of physical damage - good thing! The only caveat is that something should be done to differentiate different types of weapons. It could be as simple as having them all be very similar in effect but vary their resource costs/production time. But some weapons could have effects/abilities attached to them. Sharp weapons could cause bleeding, blunt weapons could have a chance to stun/knock back depending on the relative strengths of the opponents, etc. So long as maces and swords and spears are all ~equally worthwhile I will be happy.

Magical damage - there should be Fire, Magic, Earth, Air and maybe Life/Death types of magical damage (and associated resistances). Leaving this out would be totally counterproductive to having a complex magic system. I'd also like to see a willpower stat - it would affect morale and would come into play when dealing with mind-effecting spells.

I think morale would go a huge way to making combat interesting. There are tons of ways it could be implemented; troops could break and flee if morale goes too low, they could rally or gain a second-wind (and in generally fight better) if they have high morale, etc. But if breaking/fleeing is too complicated, severe penalties to combat effectiveness, or even freezing in terror, would be a nice consolation prize at least.

Regarding experience - I think experience should have a small effect on Attack/Defense/Willpower - these things represent combat skill and battle-hardiness. A battle-hardened warrior is not going to be as affected by adverse conditions on the battlefield as a less experienced soldier; and all else being equal, the more experienced soldier will have an advantage over the less experienced based on skill - not higher HP or things like that. IMO HP should only depend on race, equipment and magic - maybe with a few exceptions like channelers, or dragons, etc. But I think where experience really shines is with traits/abilities - ala Civ IV, basically. Like someone else mentioned Elven Legacy's system was pretty good. What traits/abilities are available could depend on faction, race, equipment (and maybe training, depending on whether or not training can provide actual abilities - like scouting, for example).