Nothing wrong with it at all. I don't understand why it would be so bad to take a world that already exists and expand on it with your own original ideas. There's no harm in it--except, of course, for those close-minded fools who can't wrap their mind around anything different. And if there are no differences, they'll say it's too much of the same. You just can't win.
Perhaps if we go back and get Misery or Harry Potter or Walden Two, etc., and change the character's names to Mario and Sonic and Donkey Kong, they'll have a lot less to read and a lot more to complain about for such a samll reason.