I think it would be neat if summoner had a buff or maybe sacrifice their summon to be able to fight similarly to yuna in ff dissidia for a certain period of time and you could alternate this with the bahamut form they already have. Be able to combo different moves from different summons in a combo instead of just having the one summon out the entire time.. would be cool.
Video Example: https://www.youtube.com/watch?v=nCUHu5802kU