Kingdom Hearts 4 features Pixar's Coco because Tetsuya Nomura thought it "would be the best world" for Sora's story
Kingdom Hearts lead Tetsuya Nomura says that Pixar's Coco was chosen as a Disney world in Kingdom Hearts 4 as it was the best fit for Sora's story.
Speaking at the Kingdom Hearts D23 panel (via Genki_JPN on Twitter), after the reveal that The Land of the Dead from Pixar's Coco is the first Disney world featured in the upcoming Square Enix RPG Nomura spoke about world selections.
"Whenever I do have a chance to decide that we absolutely need to include a certain world. I generally think about how well it synergizes with overall the main story progression with Sora" he explains. And that with Kingdom Hearts 4, Nomura adds, "when I thought about the themes, we felt that Coco would be the best world that would help achieve that goal."
Spoilers for Kingdom Hearts 3 (and potentially 4, I guess) to follow.
Okay, so from my understanding as someone who has not played Kingdom Hearts 3 yet (I'm trying to make my way through all of the non-numbered entries between 2 and 3 as intended) Sora sort of dies at the end of the game by being banished from his reality, before awakening in the realistic world seen in trailers for Kingdom Hearts 4, this is known as the Quadratum, which acts as an afterlife.
Not only that, but fans speculate Sora has lost his memories in Kingdom Hearts 4, due to him fainting when seeing visions of Goofy and Donald. This then ties into the Coco world. The latest trailer opens with Hector explaining that people disappear from the afterlife when forgotten (like in the film), which seems like a pretty intentional thing to open with, especially if this theory is true.
Part of the reason the first Kingdom Hearts is my favourite in the series is mostly down to how much the Disney side was integrated into the main plot – which is something I've seen as a major complaint with Kingdom Hearts 3 – so if that's the case again with 4, it's certainly an exciting prospect. But at the same time, I really want the most esoteric crossovers possible to happen with Disney's massive library. Give me an Arlen, Texas world where Sora teams up with Hank and Bobby Hill to stop the heartless from taking over Strickland Propane, I beg of you.