I like Diabolos, I feel like he could make a comeback. I agree with Moon though, the sort of 'game-exclusive' summons should probably remain as such, especially if they're ultimates.

Which is why I'm not sure I want the espers from FFXII returning unless it is a game set in Ivalice, I feel as if they're integral to that particular land and culture. It'd feel weird seeing them elsewhere.

I like that Carbuncle is back, though hopefully it isn't as useless as merely casting 'Reflect'.