Two characters,Spoiler:Roxas and Xion, were originally never going to appear in Kingdom Hearts III but Nomura decided to feature them in the story eventually due to him fearing massive fan backlash if he didn't.
The unique style of ending for the Spoiler:Yozora secret boss battle at the end of the ReMind DLC, where players obtain a different ending depending on if they win or lose the boss battle, was because Tetsuya Nomura and his team thought the fight was too extremely difficult and so wanted to give something to players that lost even if it did nothing for the series' canon.
The game was originally going to be run on Square Enix's Luminous Engine, but due to problems that arose during development that involved the Kingdom Hearts team not getting used to the engine in enough time, the game was moved to the much more familiar Unreal Engine 4, a move that caused a delay in development.
Pixar were heavily involved in the creation of the Toy Box and Monstropolis worlds based on their properties. They collaborated with with Square Enix on the story, character design, and look of the worlds among other things and helped to design the Infinity Badge Keyblade. They even negotiated to have the story of Toy Box be set between Toy Story 2 & Toy Story 3. This went so far as to have Pixar suggest Andy's Room have the Starfield wallpaper instead of the iconic Cloud wallpaper, so the area can be chronologically accurate to the film series.
Arendelle's Boss, the giant wolf Heartless "Skoll", is named after and based on the figure of the same name in Norse (Viking) mythology. Specifically, the Vikings believed that two wolf figures, Hati and Skoll, were in constant pursuit of the Moon and Sun in order to devour the two celestial bodies, thus creating the day and the night. Skoll can also be seen trying to darken the boss arena at times in reference to Skoll successfully devouring the Sun during the Ragnarok, the end of all of the realms of the universe in Norse mythology.
When using the Classic Tone keyblade's Shotlock, Sora actually turns into the same exact cartoony monochrome form that he had donned when he was in the retro Timeless River world in Kingdom Hearts II. In addition, the 'portals' he comes out of and goes into during the Shotlock are extremely similar to the doorways that lead to different areas of the world that are used by the characters in Timeless River's story.
During a scene with the character Demyx, he spouts the line "Yes, Demyx time!" This may be a reference to the web-series "Demyx Time", which released in the late 2000's and followed a cosplayer dressed as Demyx going over Demyx's daily life and events inside Organization XIII.
The team behind Square Enix's Einhander helped develop the expansive Gummi Ship aspect of Kingdom Hearts III. They even included references to Einhander, including an unlockable Gummi Blueprint obtained by scanning a constellation in the Misty Stream area that greatly resembles Einhander' Endymion ship. One of Einhander's bosses, the Schwarzgeist, also cameos as a boss in Kingdom Hearts III, and a remix of Einhander 'Thermosphere' plays when the boss is fought with most Gummi ships. However, if the boss is fought with the Endymion blueprint, then the original PlayStation 1 version of the Thermosphere theme will play.
Spoiler:Scala Ad Caelum, the name of the game's final climactic world, means "Stairway to Heaven" in Latin.
On the game's box-art, the clock on the tower to the left of the characters actually has thirteen roman numerals instead of twelve, and they are out of order. In addition to that, the two hands on the clock are pointing to 13 and the 7, representing the 7 lights and 13 darknesses that play a major role in this game's story. The clock tower itself bears a striking resemblance to the NTT Docomo building in Shinjuku, Japan.
In the Misty Stream area of the game, the player can find a wreckage of a pirate ship which appears to be the Phantom Storm boss from Kingdom Hearts II, s can be seen by the identical jaw piece.
According to Tetsuya Nomura, the aesthetic and environment design for part of the Hercules-based "Olympus" world was primarily inspired by California's Yosemite National Park, despite, story-wise, taking place in Ancient Greece. This decision came about when co-director Tai Yasue had visited the real-world location.
During a Square Enix Presents segment at E3 2015, Tetsuya Nomura revealed that the kingdom of Corona from Tangled was one of the first worlds to be considered for the game.
The Teacup Attraction Flow was originally going to be in a first-person perspective, however, this angle gave testers motion sickness and was changed to third-person instead.
Kingdom Hearts III is the first Kingdom Hearts game to be on a Microsoft console.