[Squirrelloid]

As long as we're tossing out suggestions...

Use the terms 'heavy' and 'light' appropriately. (Seriously, 'light' means a primary ranged weapon, not that they're wearing lighter armor - although historically these tended to go hand-in-hand because light infantry/cavalry were more cost effective without the heavier armor, its not what the terms mean). And when I use these terms below I intend them in their actual military sense.

More historical unit performance and army performance. For example:
*Sufficiently long weapons receive first strikes when charged (enemy moves up to them). Ie, pikes vs. cavalry.
*Heavy cavalry 'trample' and penetrate into units of troops they charge.
*Heavy cavalry 'cause fear' against infantry. (Undisciplined units should break and run in the face of a heavy cavalry charge, especially light infantry).
*light cavalry actually uses historical light cavalry tactics (shoot while falling back) - this may require larger battle maps. The muslim armies during the first crusade lured crusader armies into ambushes by harassing with light cavalry and then leading them in apparent retreat, often for over a mile or more - all the while firing back at the pursuers - until the pursuing crusaders were surrounded and overwhelmed by ambushing forces.
*Armies which are defending a territory entrench themselves. Archers put down archer stakes in front of their position. Units construct earthen ramparts, etc... Similarly, sieging armies of at least some nations should be able to practice circumvallation (building an earthen rampart all around the sieged city). Notably the Rome themed nations, since this was Rome's historical siege tactic.
*Cities which are starved long enough surrender.
*light infantry attempt to fall back and regroup when melee units get too close.
*light infantry receive the ability to 'Fire Rearmost'.
*Units behave more like units and less like loosely aligned mobs. In particular, they should hold formation when at all possible. (melee will necessarily involve breaks in the line and whatnot, but they should move together as a unit even if some individuals are faster than others).
*Armies should be allowed to pillage in the turn they conquer a province (seriously, its a month between turns). Ie, a 'move and pillage' command. The hundred years war was mostly a conflict of armies pillaging the countryside with the opposing side trying to chase them down and make them fight. (Ideally, moving and pillaging should not cause the territory to change hands, but it produces nothing for the owner, cannot produce units while occupied, and the pillager gains gold and gems at some % of the territories normal rate. Pillaging armies with a map speed higher than 1 should be able to ignore PD (PD is too slow to assemble to deal with an army that doesn't intend to actually fight).
*Mounted units require appropriate amounts of food. If people are size 2 and horses are size 3, each mounted person should require food for a size 2 and a size 3 individual. Similarly, they should consume air (from air producing items) appropriately. The way the game currently handles it the rider should starve, and also suffocate when brought underwater via an item which produces a fixed quantity of air.
*Ability to provide logistics for your armies without magic. Living off the local province is not the only way to feed an army without magic.