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.

Tasha Sounart, Pixar's creative director for the Disney Parks and the consultant for Toy Story elements in Kingdom Hearts III, has claimed that she believes the models for the toys in the game were the best they have ever looked in a licensed product that wasn't made in-house at Pixar:

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 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.