I'm kind of wondering if maybe Caius will weasle his way into KHIII, as Sephiroth did in the past. I don't see Lightning coming into the KH universe, but maybe some of the minor characters...Maqui or Yuj, for example. Edit: With Agito coming into play, maybe a character from the Type-0 universe? I don't know her name, but I always like the girl who was obsessed with explosives in Class...4...? She was the one that Queen, Deuce, and Cinque were gossipping about I think.