>>533699331
Coding a lot of new shit. That's the main problem

Just importing a whole KH world to another game would not work since there's a shitton of shit that needs to be coded again in the ""new game engine"" like collisions, teleports, triggers, path finders, enemy spaws, stage animations, skyboxes, npcs, interactable elements like save points, etc.
Take the Neverland world in the Nobody May Cry mod, that shit was in theory an easy world swap since they only imported the low poly-ish stage from the DS game and that whole level is basically a huge empty room with a few platforms here and there
https://x.com/Project_NMC/status/1898895397898141930
https://x.com/Project_NMC/status/1900293402182205749

Even though characters swaps are more common and easier to do in modern hardware you need to be careful or you get Lovecraftian horror tier models like pic related