Reimagining an existing fictional world needs to be done carefully. This can be someone else's intellectual property. The copyright status of the Land of Oz is complex.
Copyright status of Oz
If you work your way through this Wikipedia entry you will soon discover that various works of the Oz oeuvre is still in copyright and will be so for quite some time.
This means that you can only use whatever it is that appears in the books that are currently in the public domain. Use something from books still in copyright and trouble will ensure.
So far this answer assumes that you reimagining the Land of Oz in an open manner. So that it will be transparently and openly the Land of Oz. The alternative approach would be to take the Land of Oz as a template and rebrand as your own fantasy world, albeit an Ozesque fantasy world.
In this case, you must still tread carefully as the current copyright holders might resent interlopers. In either case, any potential publishers will have to assess whether people want to read books that are either based directly on the Land of Oz or on a fantasy world based on Oz, but may have diverged from the original template. This is difficult because there is an existing body of works in the form of a long series of books about the Land of Oz.
Tread carefully the territory belongs to other people.