Okay, I have some theories I need to get out cuz I'm fanboying hard right now about these theories! Quadratum has to be a Final Fantasy realm.* *Quadratum literally means Square.
>Yozora is obviously KH's version of FFVXIII's Noctis Lucis Caelum. Both names having similiar, basically the same meanings!! But, Yozora might be Noctis' Nobody. "This isn't what I really look like."
>Final Fantasy characters are descendants from Quadratum.* Why would they only be in contact with more powerful Disney Characters? *Merlin, Fairy Godmother, Yen Sid. And, why would Disney worlds not be allowed to know about other Disney worlds, but the FF crew does get to know about other worlds? It's a small possibility currently but, TWEWY crew might come back.* Seeing as Neku told Sora "See you in Shibuya" And Sora's literally in Shibuya. I believe that *Sora's side of the Secret Ending, is after his fight with Yozora. Sora struggles a bit to stand, looks at his hand, examines his surroundings, and then he notices the 104 building. Riku's Dream is a dream of future him in Quadratum.
>Riku has been in Sora's dream in DDD, and has always had the strongest connection to Sora. Riku stated himself "I was looking around for Sora, when I felt someone watching me from way up high." Riku's dream self KNEW Sora was in Quadratum. Even Master Yen Sid had a theory on this!!
"And because Riku had been in Sora's dreams before, he thought perhaps Riku just may be the key." - Fairy Godmother Kingdom Hearts is the heart of Disney's Universe
Everyone has a heart, even worlds. So an entire universe should have one. Kingdom Hearts has always beckoned the Guardians of light to protect it.
And my last one: The World that Never Was, might be a part of Quadratum. The look of TWTNW is similiar to Quadratum's dark, glowing, night aesthetic. Even some of the names of the areas are ironic. "Alley to Between", "Memory's Skyscraper"
The text on some of the neon signs even look similiar: