Well, tactical combat certainly needs something - there's just nothing to differentiate most units currently, and not much to do really aside from charge in and time it so you get the first hit in. More unit abilities, both passive and active ones, would fix these problems. In the spirit of custom units, I'd rather avoid Civ4-style systems like "Spearmen get +50% against Cavalry" - but it's possible to implement something similar without being so strict.
For example: spears could do double damage on counterattacks. It makes spearmen more effective at stopping any melee unit, but doesn't help them on offense or against archers, traditionally weak areas of spears. To be clear, this ability would be attached to the spear itself, so you still have full freedom in designing your custom units with the abilities you want without being restricted by unit types like "cavalry" and "spearmen."
Then you could give mounted units some kind of charge ability, that allows them to move faster without attacking faster, and maybe do double damage on the first melee attack - making them better suited to closing the distance with and quickly killing unarmored archers, but little better against well protected infantry. Again this is associated with the mount itself, you'd be free to put a spearman or archer on horseback, the charge ability just might not be as useful for them.
Next get rid of this nonsense of maces hitting twice as hard as swords, which makes the latter useless*. Instead have them do the same damage, swords attack faster and have a chance to parry (take no damage from) enemy melee attacks/counterattack, maces penetrate half your enemy's armor (i.e. they roll 0-10 instead of 0-20 defending against you). Now swords are better overall damage against some lightly armored foes, and better defense against other dangerous melees, and maces are better against anything with decent armor so long as you're not worried about them hitting you back. Make spears do equivalent damage, along with that better counterattacks ability, and voila - interesting tradeoffs, not just "pick the weapon that has the highest damage and make 20 of those guys."
Anyway, those are just examples - the main idea is to associate abilities with items, creating meaningful differences between units without needing arbitrary unit types like "cavalry" and "archers." Let players keep full freedom to design custom units, combining items and associated abilities as they like, and let rock/paper/scissors-esque balance develop naturally from the different combinations of those abilities without any artificial "+50% vs. cavalry" rules.
Unfortunately, I fear that much of this will be impossible to balance so long as we keep the "attacking faster = running faster" system. This really needs to go, how do you arrive at a good balance between slow/hard-hitting weapons vs. fast/weaker weapons when the latter makes a guy run and cast spells (!!?) faster? How do you balance fast-moving cavalry against slow-moving infantry when the former gets to attack faster too?
*Who in their right mind would want to attack 4x, triggering 4 defense rolls and up to 4 counterattacks, when they could attack 2x for double damage? Sure it's the same damage against a defenseless enemy, but as soon as you consider defense and counterattacks swords are clearly useless.
