Ariel was originally meant to be a Princess of Heart, but in order for this to happen she needed to be captured by Maleficent – and this meant that Ursula needed to give Ariel her legs. To make this happen in the story, Ariel would have had to be introduced into Kingdom Hearts already progressed in her story to the point where she had legs. The series was too far into production though to add this. Because of this the producers ended up choosing Alice to be a Princess of Heart.
Sora’s role in the series as the protagonist was originally not something that Disney wanted to do. Disney apparently wanted Donald Duck to be the protagonist, but Testuya Nomura (the series creator) wanted an original human protagonist. It was said that Nomura wasn’t interesting in making someone else’s idea and wanted to make something unique.
Kingdom Hearts producer Shinji Hashimoto is to thank for the series being created, due to him meeting with a Disney executive in an elevator. Tetsuya Nomura, at a panel for the D23 Disney fan club convention in 2017 explained, “The Square Enix Japan office happened to be in the same building where Disney’s gaming division was. It just so happened that our executive producer, Mr Shinji Hashimoto, and a Disney executive rode the elevator together. They would have conversations saying, ‘Wouldn’t it be nice for us to do something collaborative?’ So rather than having either Square Enix or Disney approach the other with a formal business plan, it was all down to being in the same building.”
The Final Fantasy characters were added in so that original characters didn’t need to be created for minor parts in the game.
The Deep Jungle world only appears in the first Kingdom Hearts game as Square Enix did not renew their license for it after the first game was completed. The rights to the original Tarzan are owned by the Edgar Rice Burroughs’ estate. Disney secured animated film rights from the estate for Tarzan back in the 90s, and therefore are allowed to do what they want with their own version of Tarzan. As Square Enix is a separate company to Disney though they most likely would have had to get permission from the Edgar Rice Burrough’s estate for a license to Tarzan. It was never revealed why Square Enix did not renew their license for Tarzan after the Kingdom Hearts game was released.