That's a good question. Let's see...
1. Yozora.
He appeared only in one cutscene, but left so many questions so I really want to learn more about him.
2. Ventus.
With recent updates of UX this character was shown in a different way than in previous games and I can't wait for the moment when his past will be revealed in the main story.
3. Master of masters.
There was time when I was waiting for him to appear in KH3. Since he didn't, all I can do is wait for next games. He's an interesting character and I wonder what he try to achieve.