Remathilis
Legend
Weeeellll....From WotC point of view, Oz is conveniently in the public domain and not subject to trademark. They could straight up do Oz if they wanted.
Book Oz is public domain. Like a lot of properties, later adaptations are still under copyright and need permission to use elements from it. So WotC could use Oz, but it cannot look like 1929 Oz with Ruby slippers or green skin witches. (Yes, they had to get permission to make Elphaba green).
That's the problem with characters and work in the public domain: so much of what we consider integral to the character isn't PD and lawyers will nail you if you step out of bounds. The more famous (and adapted) the character is, the more likely you are to step on a mine.