Interesting thread on the XOC board about Robin Hobb, fanfic writers and the associated legalities.
Thinking: I am writing a fanfic based on a film, which is based on a musical, which is based on another musical, which is based on a novel... And the novel itself is purportedly based on urban legends and possibly on other novels (the many, many similarities with various Gothic tales and even "The Hunchback of Notre Dame", a classic at the time Leroux was writing his "Phantom", do not strike me as coincidents). Maybe ultimately, everything is a little bit derivative...