Awesome game here are some of my ideas:
1. Add the Ability to use bows or ride on horseback/elephants
2.Be able to customize your character/units
3.Add a Chariot
4.Add Spawners (The player can modify the spawner they placed Ex: how many units it spawns per minute, max units it spawns etc...)
5.Add squads (Ex: a squad of 4 will stay together and follow their leader) or add batallions (You get to be the leader or another unit)