I don't remember if this was ever mentioned, but here are my hefty $0.02 of what I think would be a good setting for a MU* (or as I like to call it my dream game settings)
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The Magicians: I think that Breakbills, Breakbills South, Neitherlands, and Fillory could all make for good landscapes, but you could also just create a few other realms if you want to deviate a bit from the canon material (by way of one of gazillion fountains in the Neitherlands).
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Red Rising: If you haven't read the books, I would strongly recommend them. Awesome! It has an entire class/caste system (Society of Colors). It's one of those that would also be fun for someone looking for a sci-fi Lords and Ladies type game/concept
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Poltergeist: The Legacy (or Friday the 13th: The Series or Warehouse 13): You all play mortals or mortal+ (to use the WoD "race") traveling, investigating, protecting the world from powerful/dangerous objects.
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The Old Kingdom/Abhorsen: Two different worlds divided by one wall. A world of plain mortal men and women, and a world of magic (free-magic and charter-magic). It has a developed enough world and timeline that one could squeeze an original game setting somewhere within.
There are others I'm sure I have floating in my head, but here are some that wanted to be noted in the conversation.