Kefka as a playable character?

Possible reasons for Kefka to save the world?

A: Gesthal's dieing words, you can't rule the world if there is no world.
B: Because its easier to make people suffer if they are alive.
C: So he could kill everybody himself.

If any of those are the case, you could actually, fairly easy switch Celes for Kefka. (It wouldn't be hard to tell a story where Celes goes nuts). Locke saves Kefka (instead of Celes) in South Figaro to interrogate him. He joins the party for revenge against Celes for taking his role of crazy killer. For the Opera... eh, I could see Kefka dressing in dress and singing... or just burn the whole place down first. Get to the World of Ruin, go recruit everybody back with the ruse of "saving the world", but really just because Kefka wants to be the one doing the killing.

It could work.