If they were ever to do this and actually put it into production, I 'd like to have them get in contact with Nomura and have him draw some key art and maybe even a short(~20 seconds) animation for all of the non-Disney characters, especially the main characters. If the animators could keep to that, with some minor tweaks, you could have a true mash-up. Nomura's characters along side Disney's.
In the interview from KH Union with Kearsley, I think he even mentions about how he did not want the series to disappoint Nomura, so I assume if they went back to it, Nomura would definitely get some say, even if not the final say since it's both SE and Disney owned.