I honestly would love to see another collab of some kind. I feel like Kingdom Hearts would probably see the biggest spike in players (and honestly, you COULD do some really cool stuff, like Housing items, full glamour sets, and obviously weapons), but unless the team was PRETTY ambitious with it, I feel like you'd probably risk disappointing players.

Only exception would be if EVERY Job got a Keyblade to glamour (or other iconic weapon, like the Organization weapons?). Even if just the Kingdom Key (or Mickey's "Reverse" Kingdom Key at the end of KH1).

Otherwise, FF7 would be a pretty obvious "win" for the fans, presumably also with some weapons and maybe a mount of some kind.

For me, I would be LOVE to see some kind of a low-key collab with Mystic Quest. Maybe sort of like Hildebrand, Benjamen would be this somewhat clueless adventurer who gets in over his head. Or alternatively, he arrives in Eorzea expecting to become a great hero, only to be in awe of some of the threats we've faces, finally deciding to get back on his boat at "find someplace less dangerous to become a great hero".