they don't have the proper lisence to put films such as spirited away inside kh3...
they don't have the proper collaberation with pixar either which is why nothing from pixar has been included in the kingdom hearts game...its a really complicated thingy...
probably the reason why they don't put final fantasy worlds in there is because each final fantasy has it's own unique story. mention just one place out of that world and you would have to dullify and greatly shorten these stories and plotlines. it's not like disney movies where you can compact the plotline and just take out scenes from it. take mulan for example. look, you could gaze through the whole story within 30 minutes...and the movie was like 3 times that much. i personally wouldn't want any final fantasy worlds inside kingdom hearts 3 because they would either have to shorten the story or crop it considerably since all final fantasy stories worth including are fricken long...or to do the story actual justice, port the whole story, plot, characters over to the game itself, which is a stupid thing to do.
i don't think fantasia deserves it's own world. it would be better suited as a place you fly through on the gummi ship.