The legal process of actually procuring the rights to use characters from different shows would be an absolute nightmare for Square-Enix, so one should not expect involvement of any characters not already owned by Disney or Square-Enix.
That being said, I think it would be stiffling to add more crossovers into the Kingdom Hearts universe. There's more than enough material between Disney and Square-Enix to make use of, so I don't see a reason to throw more stuff in.
If Disney is going to play a big role again, I'm hoping that there won't be any reappearing worlds from Kingdom Hearts 1 and 2. The worlds that returned in Kingdom Hearts 2 already stretched the plots thin (or completely did away with them in some cases). I think SE would be beating a dead horse (excuse the cliche) if Agrabah or Halloween Town were to make a third appearance. The only world that is worth bringing back is Disney Castle, and that's 'cause there isn't a pre-established plot going along with it.