A good chat indeed.
To add on to your idea a bit:
Say after 10 rounds of combat your heavily armored swordsmen get "fatigue". A drawback from wearing heavy plate and they lose a 2 attack and 1 defense. Meanwhile your light spears are just fine stabbing at the enemy in their light leather.
Also you could have something related to unit size, say your pikemen are getting whittled down and they can go berserk which increases their attack by 5 but their defense drops down by 4.
The possibilities are almost endless.
