The Sherman Calliope can be almost the same as the Katyusha, but more health and deadly than the Katyusha. The Katyusha could be a normal unit while the Sherman Calliope is in the League City armaments.
Also for expanding/leveling units in the army base, they can evolve into something stronger. For ex. The jeep can turn into a 12 fold rocket carrier once it reaches level 3/5 it can still be a cut car
These are just small suggestions for the game