I checked once, and it appears as though the entire footprint of the Magic Kingdom (including a good portion of the behind the scenes sections) would fit within the footprint of Mall of America. This isn't to say that they have plans to "gut" MOA, but a similar structure could be built anywhere to adequately do the job.
While building an indoor Magic Kingdon might do little to satisfy "purists," it would probably do quite well in the colder sections of the U.S. If there's one thing that MOA, some of the huge water parks in the Wisconsin Dells, and the West Edmonton Mall have taught us is that there's a demand for such structures during those cold months we have to endure.
Just a thought...