I don't expect an answer to this, because I don't think there is one, but I'm curious about people's opinions. At what point is the switch flipped from "fanfiction" to "original work"? I know you're definitely in the former if you're using a story, setting or character from an otherwise original work of fiction. But what if you're only using a race? Or a type of robot? Or a device, like a lightsaber? When do you cross the line?
The context for why I'm asking is because I finally decided to submit my current story efforts to WFG, and one of them was rejected for being fanfic. Understand that I'm totally fine with this ruling - my only immediate worry being that moderators will think I'm an idiot for not reading the guidelines, when that's not it - I just honestly hadn't considered it as fanfic. But it's true that one of the main characters is a Time Lord, with a TARDIS. No characters of "Doctor Who" appear, the setting is definitely not the same universe (seeing as how they discuss "Doctor Who" being a TV show), and the story is my flavour of bonkers - yet one of the main characters is a Time Lord. In retrospect, I can see it, given that I didn't create that mythos, but anyone keen on reading "Doctor Who" fanfiction really doesn't have much of an entry point. (Thus I've created the worst sort of fanfiction! No one will read it!) Hence, the curiosity - if I write a story about Cylons, not cool, but if I simply change them to robots, are we all good? Is it really that easy? I wonder.