Here are my ideas: 

Monsters Inc. has a lot of DOORS. KH is all about doors. Flowmotion would be perfect for the door vault where we chase Randall. And RANDALL, PERFECT BOSS FIGHT. Team up with Boo, who has a baseball bat, to fight Randall, who can camouflage to heat things up. Then when the fights is done, we get a new summon, SULLY. I imagine he would be just like Simba, who by the I loved. And the Himalayas, perfect for a minigame: The Sled Ride. And another minigame/bossfight: Fighting the CDA like the 1000 Heartless or Card Soldiers in Wonderland. Not to mention the graphics, which would be absolutely beautiful by the way. There are so many aspects in Monsters Inc. that SCREAM (pun) KH3. 

Here are my ideas: 

Monsters Inc. has a lot of DOORS. KH is all about doors. Flowmotion would be perfect for the door vault where we chase Randall. And RANDALL, PERFECT BOSS FIGHT. Team up with Boo, who has a baseball bat, to fight Randall, who can camouflage to heat things up. Then when the fights is done, we get a new summon, SULLY. I imagine he would be just like Simba, who by the I loved. And the Himalayas, perfect for a minigame: The Sled Ride. And another minigame/bossfight: Fighting the CDA like the 1000 Heartless or Card Soldiers in Wonderland. Not to mention the graphics, which would be absolutely beautiful by the way. There are so many aspects in Monsters Inc. that SCREAM (pun) KH3. 

 

If Pixar is implemented, Monster Inc would be perfect for world travelling. Imagine gonig through doors and fighting Randall in Disney/Pixar worlds that are not part of the KH3 storyline (Carl's home from Up, The mines carts from Home On The Range, Paperman New York etc.) the possibilities are endless!  

I'd be cool to have Monster inc. in KH

I hate to be a killjoy, but I just can't picture a Monsters Inc. world. Your boss fight sounds pretty cool though!

Sounds nice and all, but I think that the antagonist is not strong enough. It is not exactly clear until half way through the movie.