I guess you could have Mickey as the main character, Donald as a Riku-type character and Minnie as a Kairi-type. Personally I'd prefer Sora, Riku and Kairi as I'm not sure how much freedom the writers would have with the Disney mascots.
I would have liked some of the original and FF characters to be replaced by Disney. I'd like a Disney villain to be Xehanort, possibly the Evil Queen from Snow White. It would be tweaked a little bit but it isn't totally strange for the Evil Queen to do most things that Xehanort has done. She was pure evil in the film. Instead of Leon/Yuffie/Aerith, they could have Prince Phillip, Prince Charming, Aurora and Snow White helping Sora. Maleficent can have the same role. This may sound odd but maybe Lady Tremaine in Ansem's role only because the Xehanort stealing Ansem's identify could work with Tremaine and Evil Queen. Obviously the story will differ slightly with these villains but not as drastically as you may think.
Organization XIII can be filled with Disney villains. Maybe not give them new names if they go the nobody route. Xigbar and Saix can be Jafar and Hades. Eraqus can be Merlin but he obviously won't die. Merlin's current role can be either the blue fairy or fairy godmother. Axel's role is a tricky one - maybe Pete can be his role and go the "bad character goes good" angle. Obviously different once again because Pete is much dumber. Pete's current role doesn't need to be replaced. You could even have Oswald the rabbit in Axel's role haha
All Sora's forms stay the same (Roxas, Xion, Namine, Ven, Vanitas) as well as Terra and Aqua. You may even need to switch Terra and Aqua's role if the Evil Queen is the big bad. Other FF characters can be replaced by random Disney characters - Cloud can be Quasimodo, Auron - Mufasa lol, the FFX fairies - Bernard and Bianca, the world ends with you teens - a group of talking animals such as Pongo, Perdita, Lady, Tramp, Oliver, Dodger, Todd, Copper, O Malley and Duchess - genius lol
sounds bizarre but with some slight tweaking I wouldn't mind it. It doesn't need to be less serious though.