I think it's reasonably safe to say that summons who have only appeared as summons in one game are unlikely to return. There may be an exception, but generally speaking it's safe to assume that most of the aeons from X and GFs from VIII won't be showing up.

I think it's safe to assume Ifrit and Shiva will show up, as they have been summons in every single game since III, which is the first one with summoning. I'd say that about Bahamut too, except that I'm given to understand that he's not a summon in XI. It is safe to say that there's a decent chance that Ramuh, Carbuncle, Fenrir, Leviathan, Odin, and Phoenix will show up since they've been summons in so many other games, as well as the summons from Both Tactics games since I guess there's a connection (never played either of them).

Neo-Bahamut and Bahamut-Zero will never show up again. If they do, I will shoot the game's director. Same thing with Knights of the Round, although I understand that one was also in VI as Crusader.