Or just add a lot more Primal fights, assuming they simply went with only previously well-known summons/eidolons/guardian forces.. the list is quite large:

Asura
Remora
Catoblepas
Ragnarok
Hades
Kajata
Brothers
Doomtrain
Eden
Pandemona
Atamos
Ark
Anima
Yojimbo
Magus Sisters

Pretty much all of the Final Fantasy 12/Tactics Summons (Although I think those would be much better done as an entire expansion. Imagine sailing to the distant lands of Ivalice to continue saving the world)
This is also excluding a couple dozen others that I feel are already in the game in a different form such as Sylph and Quezacotl (Already having Ramuh and all). Not to mention I am sure there are some young and hungry programmers with great ideas just waiting to be called on.