I think the game should add more requirement for using units. Example: add skill system, if you use thorn or defeat enemy has thorn, you gain some using thorn skill exp; if you use archer class unit or defeat enemy has them, you gain some using archery skill exp; if you want have thorn royal in your army, you need get 100k using thorn skill exp, 50k archery skills exp. The requirement
will make player using more unit than using some unit for all combat.\
I think requirement should random (ex: Black Dragon at agame require 100k giant slayer exp, another game require 50k using dragon exp, other require 200k master fire exp ) when start new game so player alway think new strategy for each game.
The achiement system should has more achievement and more level, replace the using quantity requirement by quality requirement (example instead of cast 50 fire arrow spell, the requirement should be do 100k dmg for enemy)