How about another hero? Well a balloon hero sounds good doesn't it?
I have had the idea of a zeppelin like hero in addition to the barb king and archer queen.
Health: 2000 (Level 1)
Damage: 200 per bomb (Level 1)
movement speed: 12 Compared to balloon movement speed of 8
Damage type: Area splash.
Splash range: 3 tile radius.
Air troop
I'll leave it to you to decidewhether or not you want it to have some method of attacking air troops.
Now many people will say: "Having three heroes will be too OP" but I have a solution. You should be able to have three heroes but only be able to use two at once. You could have a screen which shows you what heroes you can choose to attack/defend your base. it could work like you leave at least one hero on sleep mode or you could have an entirely different screen which allows you to "switch out" one hero with another.

