i have a lot of ideas for this game
1 add a wave mode ware you have to survive 50 waves of units
2 add armored elephants like why not armored elephants
3 add a map with lakes and if you fall in to the water you die
4 add decoys so like you place a fake blue guy and a red guy comes and attacks it
5 add a new weapon could a staff and it can shoot magic balls that hurt units
thats it for my idea list