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.