Honourable Mention #1 Other Square-Enix Properties
Other than Disney characters and worlds, the Kingdom Hearts series also plays host to a number of individuals from the Final Fantasy series, most notably Squall Leonheart from Final Fantasy VIII and a handful of characters from Final Fantasy VII. Perhaps the upcoming sequel might expand on these, featuring other cast members from the PS1 generation (such as Laguna, who was cut from Birth By Sleep) or some of its more retro entries (Setzer is the only character from Final Fantasies I through VI that has been featured thus far) Gilgamesh and Ultros would certainly make for interesting optional bosses at least, right? Would it be wishful thinking to also hope for the inclusion of characters from the likes of Chrono Trigger, Final Fantasy Tactics and Dragon Quest? Probably, but who knows.
Honourable Mention #2 Partially Featured Disney Properties
A number of classic Disney characters have been featured in the previous games in some capacity but have lacked a world specifically dedicated to them. These include the likes of Bambi and Dumbo, who have appeared only as Summons, as well as the likes of Chip and Dale, who appear as Gummi ship mechanics, Merlin, who appeared as a supporting character and the 101 Dalmatians, who appeared only as collectables in the first game. The latter two in particular are from stories that would make for engaging worlds, as Merlin is from the Sword and the Stone, Disneys take on the legend of King Arthur, whilst the 101 Dalmatians have an excellent readymade adversary for Sora and co. in the form of Cruella de Vil. Furthermore, with Disneys recent focus on a number of old and forgotten characters in the Epic Mickey series of games, the likes of Clara Cluck and Horace Horncollar (featured in Kingdom Hearts II) could be joined by other oldtime characters such as Oswald The Lucky Rabbit.