Several users from Kingdom Hearts forum Kingdom Hearts Insider recently purportedly gained access to around 2TB worth of game creation data from the upcoming Kingdom Hearts 3. Among them was evidence of a Monsters, Inc. world to be included as a Disney world in the game.
Below are some screenshots of the data, which were posted on the forum:
Keep in mind that this should not be taken at face value in lieu of official confirmation.
Kingdom Hearts III is due for a 2018 release on PlayStation 4 and Xbox One.