There are lots of different fantasy worlds depicted in fiction. In general, due to copyright, a license is required from the original creator of the fiction to publish a new story set in that universe (for example, you need to get LucasArts to agree to make a new Star Wars book). Are there any examples of fantasy worlds where this is not true?
closed as not constructive by Neil Fein♦ Dec 22 '12 at 19:57
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Yes, the Man-Kzin Wars represents one such world. The series started as a short story by Larry Niven, but several more stories have sprung up mostly written by people who are not Larry Niven.
I have also read books by Mercedes Lackey that are collections of short stories written in her Valdamar world by other authors.
I think if you have a particular universe that you want to use for a story, you need to do some research and see if that author or publisher allows for other people to contribute to that universe.