I personally have some plot or story ideas for KHIII.
First, if it is a continuation, we should tie up some lingering storylines. For example, there should be some kind of resolution between the somewhat romantic angle between Sora and Kairi.
As for new storylines, here are some ideas:
1- The Seven Princesses of Heart: What if it turns out that Wendy is actually a half-blood maiden?
2-A New Threat- Instead of the Heartless, it turns out there is a new source of evil helping the Disney Villans.
LIST OF WORLDS I WOULD PERSONALLY LIKE TO SEE:
HOLLYWOOD/TOONTOWN- (Who Framed Roger Rabbit?)
NEVERLAND- (Peter Pan; this time the actual island is in trouble)
BERT'S PAINTING (Mary Poppins; Sora and Co. could be on "Jolly Holiday" with Mary and Bert, until trouble forces Sora into battle.)
THE NEW WORLD (Pocahantas)
Just some ideas- give me your own two cents!