In 2024, a demo build from Kingdom Hearts III's May 2018 Premium Showcase was leaked online. Within the code, a list of worlds featured in the game was discovered, revealing that a world based on the 2006 Pixar film Cars was planned, but did not appear in the final game.

In the climax of Kingdom Hearts III, Spoiler:Donald Duck uses Zettaflare, a previously unseen spell within either Kingdom Hearts or Final Fantasy, powerful enough to destroy a solar system. This made Spoiler:the irritable cartoon waterfowl the most powerful mage in the Final Fantasy franchise at the time, to the humor and ire of various players.

In Final Fantasy XVI, Spoiler:Bahamut, who after Spoiler:Donald's use of Zettaflare became the former most powerful mage in the Final Fantasy series, would use Zettaflare during its battle in the game, regaining its former title.

Tetsuya Nomura had requested for the character Oswald the Lucky Rabbit to be used in Kingdom Hearts III, but Disney responded that he would be "too difficult" to use, though they never clarified why. While his plans for Oswald had he been added to the game are unknown, it's worth noting that Nomura has admitted that Oswald is one of his favorite Disney characters.

Pixar's higher-ups almost rejected the prospect of including Pixar content in Kingdom Hearts III, as none of them were gamers or knew what Kingdom Hearts is or how popular it was. Theme park creative director Tasha Sounart managed to convince them on how popular Kingdom Hearts is, and pointed out that catering to the series' teen audience lined up with Pixar's desire to be seen as an all-ages film studio instead of simply being perceived as targeting children and their parents alone. Sounart noted that, while the Pixar studio members familiar with Kingdom Hearts did not need to extensively detail the lore to those that weren't except when needed, it was needed to be clarified that the Pixar characters would not be meeting one another, or any other Disney characters aside from Donald and Goofy, as Pixar has an aversion to crossovers and shared universes outside of their signature Easter eggs.

In Twilight Town, to enter the Le Grand Bistrot cooking minigames themed after the Pixar film "Ratatouille", you have to interact with a book outside the restaurant. The menu that appears before entering the restaurant features the film's main protagonist Remy emerging and standing in front of a page that features a message written in French on it. When translated to English, it reads:


This appears to be a shortened, paraphrased version of a restaurant review written by the critic Anton Ego towards the end of the original film.