We had a similar brainstorm idea but chose a different one, but I'm really glad someone did it. I like it and even believe that this concept may have a commercial future.
Even though the game is short (and too short If you use bug: when you press concrete button all your heroes respawn),  It's fun. Heroes management and ability to combine them is cool, the only thing that I'd like to buy more than 1 hero each round. 
Next statements are only my humble opinion.
Pros:

• The ability to kite enemies with a melee hero during range heroes attack. It reminds me of micromanagement from RTS(Starcraft1/2). If I were you, I'd like to emphasize this feature and make it so that the player needs to do micro management more. Really like this.
• Clear visuals and user-friendly interface.
• Convenient controls.

Cons:

• No sound settings. This is a very convenient feature that is easy to do and very useful for players.
• Economy is not balanced at all, there is no risk. It's not really a con because It doesn't spoil overall experience for this short amount of time.
• Too easy last boss, not very interesting.

On point. I love this a lot. I was so happy when i realised you could merge them, but so sad when i lost them. Maybe having a set of tries ? like 3 lives to defeat the level before game over.

I liked the concept, but found it exceptionally hard to make progress.  I'm not sure if I'm meant to keep controlling them after the initial attack request.