I also think it should be more things to buy using crowns but instant healing could make it overpower and Im not sure about flying units since, although the fantasy aspects, this game has a realistic look for me. But could be a nice idea. Maybe instead of instant healing we could deploy a healing fountain on the battlefield wich heals constantly without loosing stamina.
I also think that all those things must be something that is not permanet. In units case, for example, you buy them and soon or later they will die. Buildings and new traps should be destructible forever, not like other structures that you cant destroy it and all you have to do is repair it. For example, after you have your third wall destroyed you have to buy and build another.
In addition, just a small thought:
-we could have a castle/keep/fortress with our hero or city's advisor inside in siege mode and attackers should kill him to win.