I'd like to suggest a system that incorporates some of what's already been suggested in this thread, while also incorporating some changes to make weapon type matter more and deal with some of the issues that have been raised regarding the usefulness of essence to melee sovereigns.
Action Points - Each unit gets a set number of action points to spend during their turn. The number is species-dependent, but unrelated to training or armor. I'll put base numbers in for this entire post, but they're only for illustration purposes and would be changed to achieve good balance. Let's say humans get 50 action points per tactical turn, trogs get 60, urxen get 40, dragons 100.
Action cost - Every tactical option would have a base cost associated with it. For attacking with a weapon, it would be based on the weapon type. For magic, it would depend on the spell you were casting. Each special ability would have its own base cost. Movement base cost would depend on the characteristics of the square you were moving into (i.e. forest, mud, plains). So, if you wanted to hit with a dagger, it would cost 10 AP, a sword would cost 15 AP. Moving into a plains square would cost 20 AP, moving into a forest square would cost 25 AP.
Some abilities, spells, attacks would be usable to retaliate during your opponents turn, provided you had researched the necessary warfare technology to entrench, retaliate, riposte, whatever, and provided you had ended your turn with enough AP remaining. So, a unit who chose to entrench and spend no AP in a turn could retaliate against more attackers than a unit that had moved across the board that turn.
Modifiers - Here's the part where things get interesting. Each action cost is subject to modifiers based on the unit's statistics. So, each additional point in dexterity decreases the AP cost of all edged weapons by a percentage. Strength provides a percent discount in the AP cost of using edged weapons. Intelligence decreases the AP cost of spellcasting, so more intelligent mages can cast more spells per turn. Movement speed provides a percent discount in the AP cost of movement.
So what about essence, which has been previously useless to non-casters? Essence provides a percent decrease in the AP cost of everything, after all other modifiers have been considered. So, now there's a reason for combat sovereigns to keep their essence, because as long as they have it, they move farther, attack more times per tactical turn. It also provides a reason to imbue combat, non-caster heroes, because they're better, more heroic, special, even if they aren't casting.
Weapon choice becomes more interesting as well, since there's a compelling reason to sometimes want to be able to mount several small attacks against multiple opponents versus one huge hit with a huge AP cost.
Armor could also be included in this system as a modifier, where heavier armor provides a % increase in all AP costs.
All of these calculations should be invisible to those who don't want to know the math, they just know that each action on the combat field has an AP cost. It allows you to have fast cavalry because of their movement modifier, but they can only attack more often if they are also dexterous or have fast weapons. There's only one stat for players to track, which is how many AP each unit has left that turn. It would be useful to have a button to allow a unit to save a set number of AP at the end of the turn if you're trying to save enough for a specific retaliation type.