Pixar's presence in Kingdom Hearts 3 won't be limited to the Toy Story crew. Square Enix and Disney have revealed that Monsters Inc will play a role, too. You'll see Boo, Mike and Sully (minus the famous actors, of course) as well as settings from the classic CG movie, such as the door vault. There's even the chance to wield weapons themed around the movie... including Mike, apparently.
Source: Kingdom Hearts (Twitter), (YouTube)