While I don't think copy/pasting another game's combat system is the real solution here, I think a good way to go about things would perhaps be to instigate two changes that interact with eachother for tactical combat.
I: Unit Action Point Differentiation
Units with progressively heavier armor need to have penalties to their action points. This simulates them being slower on their feet and slower to attack. An unmounted, lightly armored (or unarmored) skirmisher might be able to move three, or even four spaces on his turn. A heavily armored knight in full plate? Maybe one. With a single swing from a powerful weapon. If that. Weapons too, need to affect this, to get away from the peasants-with-warhammers thing.
This does two things:
1: It encourages things like cavalry. The bonuses a horse provide to a heavily armored soldier should more than negate the movement penalty, turning them into a mobile, mounted terror that isn't clumsy and exhausted by the time he actually reaches his foes.
2: It spreads out combat speeds nicely, for my second point. At the moment, humanoid units are too same-y for my idea to work.
II: Unit Attack Order is by Total Amount of Action Points.
Basically, instead of it being "my units have turns, then your units do," all units are placed into a pool and each get an individual turn based on their amount of action points. Units with a higher amount of points get put on the move "queue" quicker, so not only are they more mobile, they can use their abilities more often.
In practical terms, that means that a lightly armored foe would be able to outmaneuver and even outflank a heavily armored opponent to get to the meaty bits, and it also means that combat doesn't devolve, like it does now, into figuring out the most effecient way to eliminate your entire opponent's army in a single turn, while minimizing their chances of retaliation. However, the lightly armored/armed unit would still have to face the counterattack of the heavily armed, armored knight, so it doesn't have a free pass.
Here is an example to how this whole thing would play out. Your opponent has a line of longbowmen (Bows would need some rebalancing, perhaps taking twice the amount of AP currently to fire). These longbowmen, still terribly powerful as they, due to their light armor, get to take their turns very quickly compared to your heavily armored, slower footman, will eventually tear your main force to pieces before you even can get them close enough to do damage.
However, you were smart enough to include a few parties of very lightly armored, mounted skirmishers in your army!
The skirmishers, armed with damaging close-range weapons, are even more mobile, with more action points, than the longbowmen, and are able to close in and wreak havoc on the line of archers before they are cut down themselves!
This is just an example, but it's just scraping the surface of the kinds of situations you'd have to account for. Trained horse archers would be absolute terrors of course, but perhaps a training multiplier could exist that makes them extremely expensive for their relative attack power, kind of financially offsetting their abilities.
These changes not only open up a lot of tactical possibilities in combat, it makes the whole process more fair and, as a whole, engaging.
