I too agree with lswallie on most of his points. While, there's no doubt, fixes for actual bugs or anyway very strange behaviours should have the priority in the immediate, what really is lacking for me is content, and how it is amalgamated. Right now many game elements dont' really live up to the expectations or appear to be a bit chaotic.
SPELL SYSTEM: While we need more, differentiated spells and effects (more spells, yes, but elemental resistances and more effects related to a certain element would be a good and quick starting point to make them feel really different, not just a choice based on which shards you control), we also need to be GREATLY limited. Having access to all the elemental books from the beginning, plus very soon all the others, is overkill, and makes every game feel the same. Expanding each spellbook making it more self sufficient while making it so it is impossible to "own" every book will help a lot on that department and would let us customize our kingdom/empire-sovereign combo to something with lore. I would love, for instance, to use Tarth with a Summoning spellbook that would preclude access to common elemental books. Wanna play as a druid/summoner bringing forth armies of the forest? Ok, but no fireballs for you.
With some fantasy each single spellbook could be made to be as said partially self sufficient (simple example to give the idea: fire book, spell Cauterize --> heals tot HP and stops a bleeding effect, but will cost the target unit 1 permanent HP on his max HP value), letting us specialize and give some character to our realm and dinasty.
Access to other spellbooks could be binded to actual dinasty interactions, thus making 'em hero linked rather than realm linked: my dinasty has access only to the fire spellbook, but if my son marries the daughter of a dinasty which uses the air spellbook, my grandchild will have a chance of inheriting access to BOTH.
CITY BUILDING: This was already well explained by lswallie. While at growing population we can sort of specialize cities, which is nice, there could be a lot to be improved on that department. I would actually make most improvements resource dependant: cities already are used as a node point to "distribute" resources they have in their zone of influence. Just make it so if they have no direct access to metals (and maybe to a minimum quantity of metal), for instance, they can not build mining guilds, blacksmiths, etc. Then, also link certain improvements with units you can train there: a city without metal, and as such without blacksmiths, will not be able to train units in metal armors. If the city, however, has access to the old forest resource, then it could train archers with special bows.
Seems actually a rather obvious thing to implement.
TECH TREES: Same basic concept issue here: there are quite some techs (though there could be much more, especially weapon related), but in the end you research quite everything. You actually have to. Again, expanding those trees while making it harder to research more fields will do a lot to make each game feel different offering different approaches.
You want to concentrate a lot on civic research? You'll get the upper hand in population, but that should actually give you other uses rather than just rushing to the top level buildings. Like, militia units: very cheap and rather weak units, unable to compete against real military ones, yet trainable quickly in large numbers munching at your wide population.
You want instead to concentrate on magic? Make it so that would preclude you most of the more advanced military techs. No great armies of knights in shining armor then, we are a magical kingdom... with magic techs... which let us train golems, undeads, spirits... you got the idea.
A huge step forward this would be at making the game feel varied.
ITEMS: Yep, we need many more with many more new related effects. Both for actually having a reason to design troops: for building specialized units and also just for the looks, how we did in Galciv wasting time just to recreate that pretty spaceship from that pretty tv series. Naturally, the new combat effects should be related to the expansion of tech trees, spellbooks, and available buildings in a certain city, making them intertwine with relevant counter spells, specialized techs and so on.
PACING: Definitely, I beg for a marathon gamespeed. And overall for a rebalance on how the games goes on. Right now once you are done with the very first phase of the game, exploring a bit for your surroundings, then you can just turtle for a while in your cities, and zap through the tech trees at an alarming speed even on epic, practically bypassing most of the supposed mid game content.
A whole lot of new effects, resistances, spells, however, would already help themselves a bit on this department: once it's not just brute force what matters in a battle, but actual tactics related to certain special abilities, then you are not anymore so pressed on just quickly researching "the best armor and the best weapon".
DIPLOMACY: Needs a major overhaul. More things to exchange and trade for (maps, contact with other realms, units... just for starters), and definitely, please, a bit more variety in text. I always hate when in 4x games diplomacy feels just a lifeless mechanic. When I play these games I try to feel involved, to "roleplay" a story... a small bunch of generic lines, always the same, which don't adapt to the current situation is seriously hindering for that goal.
Also, the Dynasty system could be expanded on a bit. Some events related to it, that could bring forth real diplomatic consequences (not just an anonymous + on a diplomacy chart) would be welcomed.
UNITS: More different creatures are definitely needed. Not just a dozen or so, sometimes multiplied x 4 elements/texture color to make it seem there are more. And they should serve more than just as a danger for your assets and as an XP farm. Enable actual drops from battles. If I can find magic items inside abandoned bandit camps, why not in the hands of actually living - and kicking - ones?
Champions too need an overhaul to actually feel useful. Right now you just take 'em for their bonuses in the early game, then park 'em in cities. Forever. Experience should make it so they can actually grow up to be better than the late game common units you can train, as long as you made them fight and learn a lot. Again, here too several additional special skills/effects only accessible to champions on leveling up would do wonders (age of wonders, actually, if you get what I mean 
) at making 'em more useful and meaningful.
THE FRICKIN' SEA, CAPTAIN! : Bring life to the seas. Life was born there. Let life return to her cradle, hmm?
EVENTS: As well as quests, but the scaffolding for those is already in so it's natural to expect an expansion on that department. What would be a welcome addition are random events like we had in Galciv. Earthquakes, vulcanoes, plagues, sudden diplomatic incidents, rebellions, monster infestations, to mention some obvious "bad" random events. Discovery of healing herbs that raise HP regen, to mention the first idea for a "good" event that springs to mind.
MAP INTERACTIVITY: Sometimes related to events, sometimes to spell, but anyway some more visual life to the map (both world and tactical maps) could do wonders at making the game have that little more charm. Like, you know, in a tactical battle actually render the city walls when you are assaulting a city? Is it a too groundbreaking idea? Maybe even make it so walls actually serve as a defensive obstacle against enemy arrows, instead of just adding a global passive bonus to the HP of units stationed in cities? With bonus instead for archers standing on higher ground, in this case because of a "city wall" object/tile? Revolutionary? 
