I think fan-fiction can be, in some ways at least, Easier than more fully original works. But thats just because somebody else did parts of the work for you (developing a world, a character, etc). That doesnt make it anything less. Shared worlds are common in fiction, so are shared characters. Look at any of the Star Trek, Star Wars, Forgotten Realms, Hardy Boys, or Darkover novels, or any historical fiction for that matter; every one of them borrows from existing sources for characters, setting, or both.
To my way of thinking, most fiction outside of the Sci-fi/Fantasy Genre falls into that category to some degree, because they all borrow an existing world for the setting (IE the "real world") rather than make their own from Scratch. If I want to tell a story from a thief's POV, how does it make the story less legit by setting it in Mos Eisley (star wars reference) instead of the streets of New York City? My answer is that it doesn't.
The flip side of that is that you want to still be contributing. But the more you borrow, the less of an origianl work it is by nature. A story written in the DV but in an unrelated time and place it one thing. Or if its contemporary, but only mentions events in the background, thats perfectly fine, you are just borrowing the world. On the other hand if its a story about Harry and Murphy fighting Vampires, Im just adding events to an existing Character set, world setting and system; im just contributing the episode of the week. Somewhere in the middle would be if it was taking minor but established character and putting them center stage, like the adventures of Susan or Carlos or Elaine, or McCoy's childhood or something.
