Kingdom Hearts 3: 13 Ways To Include Every Pixar Movie
7. Inside Out
How: A world.
Considering the fundamental conceit that Joy, Sadness, and all the other emotions we came to know in last year's big hit exist inside a young girl's head, it's easy to see why it would be hard to include them in a Kingdom Hearts game. Summoning them on the battlefield wouldn't be appropriate, and cameos would require too many hoops to be jumped through for it to make sense.
Despite all that, a world based on the film could still work, wherein someone's mind is being corrupted from within by Heartless, requiring Sora and the gang to enter it - hey, crazier things have happened in this series - only to team up with the emotions and Bing Bong to help stave off everything from being destroyed. It could retain the essence of the film, especially if the group is saving one of their friend's minds, like Kairi, and rescuing core memories from the Heartless could keep the narrative drive moving in the absence of a film villain.