Given that Disney has a good relationship with Studio Ghibli (they are the distributors of the Studio's films internationally,) and that Square Enix has a past in working with the Studio, it doesn't seem so far-fetched to see in the future a inclusion of a Studio Ghibli world(s) in KH. I really believe they would fit in well into the franchise and it would be a good opportunity for Studio Ghibli to see their beautiful films represented in a different medium with care and attention to detail.
I honestly don't know why it was never really considered, or if it had at one point why it would be turned down. It looks at least for the moment a exciting potential prospect for the future. KH as a franchise has really grown recently so it seems like all of Disney, Square and Studio Ghibli would benefit. Please let me know what you think and which worlds or characters from Ghibli would be good inclusions!