fourth class of units
there are offense, defense, and recon units so far
maybe a 4th class? or even 5th?
there can be some type of healing class units that heal a % of your wounded units that would normally die in battle
and they are the last ones to die.
there can be armor units that deal no damage, but they take the front lines and have a huge damage absorption.
for example there can be a metal-coated heavy infantry that could absorb 600 damage but deal nothing to the attacker