Dream drop had SE characters in it from The World Ends With You. It's kind of KH's thing to feature SE characters. Tifa was already in KH in her Advent children outfit so I doubt they'll put her in her old outfit.
Lightning will probably show up somewhere in KH3 since she had so many games, she's kind of a bigger deal in the FF universe so I wouldn't be surprised if several XIII characters are in KH3.
I don't want Tifa to wear her Advent Children outfit again. That outfit is uuuuuggggglllllyyyyy. LOL. I would prefer a brand new outfit, but if not, then either her classic outfit or her Crisis Core outfit.
And Lightning, Serah Farron, Yuna, Rikku and Paine too.