I'm still betting the game is going to go longer than three installments, not to mention I can see them adding another Bahamut or two. Honestly, I prefer the symbolism of having Titan be Part 1's Ultimate Summon over just hamfisting in Bahamut and Leviathan.

My guess for part 2 is that you'll likely not have full access to all of your summons for story reasons, and then you'll do a side quest with the materia researcher that will restore all of them to you once you leave Kalm. Major materia you didn't pick up from Part 1 will likely just be given to you at this point or can be purchased from him.