I've posted it before but I think it would be cool to get the other 3 core primals introduced in ARR
It would give Summoner every element (even if elements don't mean much these days)

Upon using Bahamut:
Ifrit - Fire
Titan - Earth
Garuda - Wind

then upon using Phoenix:
Leviathan - Water
Ramuh - Lightning
Shiva - Ice

though I'm not sure what would be fitting beyond that
Solar Bahamut is a weird choice
Final Fantasy has so many different variations of Bahamut that you could make summoner use exclusively him if they really wanted to