I'd like to see them bring back the three story line game play that they had in Birth By Sleep, using Sora, Kairi & Riku with each having their own party. Sora's will continue being Mickey, Donald & Goofy. For Kairi, I would use the female counterparts, Minnie Mouse, Daisy Duck & Clarabelle Cow. For Riku's it was a little tricky, I thought about just doing Pete, but since Oswald is now part of the Disney canon, I thought about Oswald, then I found out that before Goofy came along, Horace Horsecollar was both Mickie's sidekick and Clarabelle's boyfriend, so I figured I could go with friendly rivalries using Oswald and Horace, but I couldn't find one for Donald, but then I found out about Peter Pig being in Donald's first cartoon, and how in the comics, he usually played Donald's sidekick until Huey, Dewey & Louie came along.
In short, for Riku's party I would choose Oswald, Peter Pig & Horace Horsecollar.
Also, borrowing an idea from Sonic Heroes, I would also set the difficulty based off whichever party you selected, Kairi, Minnie, Daisy & Clarabelle would be the easy; Sora, Mickey, Donald & Goofy would be normal; and Riku, Oswald, Peter & Horace would be the hard.
I would also like to see Ventus, Aqua, Terra, Roxas, Namine, and, if he has one, Riku's nobody, join each of the teams, the Nobodies would only be played in special levels that Sora, Kairi & Riku would not be able to access, and Ventus, Aqua and Terra could help train their respective team members.
It would also be interesting to see other party specific characters around the area, like having Huey, Dewey & Louie running the shops for Sora's party; Daisy's nieces April, May & June, running the shops for Kairi's party & Peter's cousins, Disney's Three Little Pigs. running the shops for Riku's gang. Maybe, they can even have Tinker Bell, and some other small character take care of Kairi and Riku's journals.
Maybe they can each be assisted by their own Final Fantasy character, i.e. Squall, Yufi & Cloud.
Also, with four members in each team, instead of three, they can make the game a four way, multi-player game