This is for characters who have not yet appeared in a KH game. Here's a few I'd like to see:
Oswald the Lucky Rabbit
Yuna (not as a pixie but the FF X-2 version)
Rikku (same as with Yuna)
The kingdom hearts series is my favorite of all time and the Star Wars movies are my favorite movies ever so I would love to see a clash of those two. With that, I would love to see Darth Vader as a boss in KH3!
Let's include boss characters in this discussion:
Clu (from TRON: Legacy)