Here is a list of characters that I think should deserve some development in character.
Sora: He should probably become more understanding on what's going on, and what other characters are talking about. What does that mean this, what do you mean that. That trait is starting to become too predictable if you ask me.
Terra: If he ever leaves his armor and gets his body back. Terra needs to take some time getting to know someone before trusting him/her. And if Kung Fu Panda 2 taught me anything, this guy has to achieve inner peace.
Riku: From what I can tell, he's becoming more like Ratchet in the Ratchet & Clank Future saga. And if the KHIII includes the world of Frozen, Riku can possibly relate to Anna.
Kairi: I wonder how she feels about her tendency to get kidnapped. That could be partially the reason why she went to Yen Sid's Tower willingly.
The Gullwings: Seems to me that their more interested in treasure than anything else. Maybe those three can get the "Ascended extra" treatment in the upcoming game. Check TV tropes on that category. And who knows, they might end up befriending Kairi. Between you and me, Tara Strong is the reason why I want the Gullwings to be in KHIII.
Tifa: She probably needs to improve her own intellect. She should know better that Cloud wouldn't fit behind a painting, or in a book shelf. And she wasn't specific to most of the known people she encounters.
Seifer and his gang: Those guys seem to be total jerks. Not to mention they seem to pick a fight with everybody they meet.
The World Ends With You gang: Maybe these five characters have a lot to learn about friendship and trust. Three of them seem to trust total strangers more than Joshua. Speaking of Joshua, I wonder how they'll react if they find out that he's duping them. Not to mention that Rhyme needs to realize that whatever she has forgotten could be important memories.
That's all I can think of.