I would love to see Rinoa appear in KHIII! I'd also like to see Ashe or Balthier, and any of the XIII crew. While Lightning is probably most likely, I think any of the others could conceivably work, especially Hope or Vanille. Hahaha, can you imagine if Superpope showed up? I'm pretty sure there's a 0% likelihood of that happening but I'd probably spend the entire time alternating between laughter and NOT YOU AGAIN >8|
As for worlds, Taaaaaangled. Oh, and Princess and the Frog! Well. New Orleans.
(Also Kairi as a playable character would be really great, Square. Just, you know, putting it out there.)