Bringing up new character ideas is fine and dandy, but its only worth it if it's handled in a better way than the KH series has done so far. I'm talking about the party system here. It needs a major revamp in my opinion. THAT is one of the things on the top of my KH3 wishlist.
Lets look at KH1 and KH2's party system for comparison. It generally consists of playing as Sora with Donald and Goofy as permanent party members and sometimes having one world specific party member who can be swapped out with either Donald or Goofy. I feel this worked great in KH1, but in KH2 there were many times when I felt that I should be able to have all 3 party members follow Sora around at once. For example: in Port Royal it would have been nice to have Donald Goofy and Jack all at the same time.
I also felt that many worlds were being limited by only having 1 world specific party member. Looking again at Port Royal, Jack Sparrow AND Will Turner should both have been party members. Similarly, in The World That Never Was, Riku AND King Mickey should both have been selectable party members. The inclusion of more than one special party member further reinforces the need to have a maximum party size larger than 2.
So this brings us to KH3. The capabilities of the PS4 should leave NO excuse for why the game cannot handle a party consisting of Sora + 4 members AS A MINIMUM. Even more than 4 would be ideal. Gameplay wise, proper balancing of party member combat effectiveness and enemy/boss strengths should compensate for having a larger party.
It's a pretty good bet that KH3 will involve some kind of "team-up" of the 7 Guardians of Light at various points throughout the story. It would be a shame if they did not end up being proper party members. Personally I think it would be awesome to have a party that consisted of Sora, Riku, Roxas, Aqua, and Terra (many other combinations are available).
The same applies to Disney world specific party members as well. More than 1 additional party member provides for the appearance of a world that is more "alive". I'll throw a couple of examples of how this would work with worlds people seem to want most in KH3. (Keep in mind, I'm not posting about what worlds SHOULD be in KH3, I'm just using the following as examples).
-Jack, Will, and Elizabeth in a world based on Pirates of the Caribbean 2 & 3
-Anna, Kristoff, and Elsa in a world based on Frozen
-Ralph, Felix, and Vanellope in a world based on Wreck it Ralph
-Rapunzel and Flynn in a world based on Tangled
-Woody and Buzz in a world based on Toy Story
-ect, ect, ect....
The same can apply to pretty much any world that SE decides to include in KH3, because most source material has more than just one hero worthy of being a party member.
Numerous other improvements can be made to the gameplay mechanics of KH3, but I feel that this is one of the most important.