I feel like Kingdom Hearts is probably off the cards unless Disney gives the okay, even if it's just using some music directly ripped from the games.
Personally I'd like a track or two from The World Ends With You. Half the cast look like KH's cast anyway!
Kingdom Hearts music probably won't happen, but I'm surprised they went with Cloud's AC design rather than the KH1 one - its appearance in Dissidia shows they can use it without any additional issues with Disney, and from a visual standpoint it's much more of a departure from the regular costume, as well as being a better fit for Smash Bros.' art direction.
TWEWY music would be great as well, and they could even use the models from KH3D to provide trophies for the main cast. Though speaking of which, I'll be really disappointed if the only Mii costumes we get from the SE collaboration are the Chocobo ones - especially if they look outside FF, there's a lot of characters that wouldn't look bad with Mii proportions.
Though as much as I find the Mii costumes based on characters in the main roster to be pointless, I'd love a Mii Swordfighter costume based on Cloud's PS1 field model, similar to the Akira costume.