That world belongs to her. YOU can find examples of other worlds that have allowed multiple writers but that is more the exception rather that rule. One thing all your examples have in common is that they were not originally book series. Lucas may have allowed people to write books in his universe but he did not allow them to write movies, at least not until then sent him a truck load of money.

Most really good writers would not want to write in someone else's world, they want to create their own. I doubt even Sanderson would want to write more WOT books.