I'd just like to point out that if you talk to the Astrologer in Windurst, he shows you a map of the Star System of Vana Diel. In this map you can see the 8 Celestial Avatars (Ifrit, Shiva, Garuda, Titan, Ramuh, Leviathan, Alexander, and Odin). On this same map you can also see several other commonly-known Final Fantasy Creatures including the Moogle, the Chocobo, and the Cactuar.
As was stated before, there are also the 5 Terrestrial Avatars, Carbuncle, Fenrir, Pheonix, Diablos, and Bahamut.
The 5 Bolded Names are the most likely choice(s) for any new avatars that would be added to the game. I personally think that Pheonix and Diablos will be added, and it will require at least some of the CoP Missions completed (likely at least past the Diablos BC, and perhaps further for Pheonix if added).
For reference here is the chart:
