I think Ultimecia or X-Death would make more sense than Kefka, He's a bit too much of a psychopath to actually manage to unite all of the major Final Fantasy antagonists. Ultimecia could appear using Kuja as a host body (Kuja was the main bad guy in FFIX, those who have played the game might get the twisted joke behind this), with Sephiroth taking over Seifer's role as her 'Knight'.

I always thought I would be cool if Square decided to make a game like this. They won't though...