I like the idea of the Foretellers as superbosses. After all, superbosses don't have to be canonical in nature, which means they could fit in just as snugly and nicely into the game as any others. Besides, they were faction leaders and as such must have possessed extraordinary abilities.
It hasn't been said before, but I personally would really love it if we could fight Sora himself in his strongest.
Fighting Sora somehow would be incredible!
I always thought that the two carnotaurs from Dinosaur would be great for a secret boss. You would have to fight the two of them at the same time. I also want either Magus from Chrono Trigger or Lynx from Chrono Cross. I think the Chrono games deserve a presence in Kingdom Hearts.
Oh man, Dinosaur is so underrated. I would love to see the carnataurs as bosses.