Kingdom Hearts 3 [Gameplay Trailer]
All I can say is since the first game was released, Disney has acquired The Muppets, Pixar, Marvel and LucasFilm (Star Wars, Indiana Jones), and Square-Enix has acquired Enix (as remember this was a Squaresoft game when it first came out), Gangan Comics (FullMetal Alchemist, Soul Eater, Durarara!!), Taito (Space Invaders, Bubble Bobble) and Edios Interactive (Tomb Raider, Hitman, Deus Ex).
I'm not saying they need to give these other franchises their own worlds like they do for the main Disney ones, I just want to see small cameos in the same way that we see Squall, Cloud, Auron and others in the game (they don't even have to be as integral as those characters, shoot Vivi and Setzer had really only a few minutes of on screen time in the series) and seeing as characters from The World Ends With You show up in the 3DS game, there aren't any restrictions on who they can have appear.
Shoot these cameos could just be in the form of items for all I care, because lets be honest, despite how great the gameplay and the story is, we all really only bought into this series because it crossed Disney and Square Enix franchises.