I think that it would be a great idea to have Alliance PvP Battles. Here is how it would work:
You would click on a fellow alliance member, there would be an option/request to battle them, like a friendly battle, then you would pick your heroes, just like you would attacking a Titan, going to war with another alliance, or going through provinces. They would accept your request for battles and you would fight. After the battle, there would be an option to watch it and to strategize with your fellow alliance member to see where you could double up on colors or level up your players or switch out certain ones that will help you get stronger.
I think this is a great alliance-building tactic and also keep people playing this game and be more active in the alliance and want to stay in the alliance.