So i'm not sure if new hero's have actually been ruled out or not but here goes anyways. First of all i think most higher level players can agree that defense needs some help and that elixir is almost completely worthless and we need something to spend it on. I propose at TH10 the dragon hero becomes available and the cost to upgrade is elixir.
The most important aspect of adding a new hero would be ensuring that attack/defense balance isn't compromised so the most logical solution would be that only 2/3 would be active at a time for both offence and defense(which would be an easy toggle feature). A flying hero would add a completely new dynamic to the game both on offence and defense while not drastically affecting the overall balance. Additionally by having a shelved hero would give you the option to upgrade one and still have 2 available.
The reason I chose the Drag is because i think the flying aspect is important and secondly whatever special attack the dragon gets would be awesome. As far as upgrades go a similar scale to the other hero's could probably be applied.



