I like the concept. I think it would be neat if later into the game there were some special enemies, like maybe an enemy that moves 1/2 the speed but takes 2 hits to kill. If you're taking suggestions on heroes to add next, how about Korra from Avatar: The Legend of Korra. Her ability could be to make a wall that could absorb a limited number of hits, but lasted forever until it did wear out.
when do you expect to release the next update, or do you not know?
Also, I have a couple more ideas: an enemy type that moves 1.5x as fast, but instead of damaging, they just disable/put on cooldown the hit heroine's power. Or, an enemy type that damages a random heroine if you touch it with your mouse (or maybe only when you're not using a power if that's too OP).