I've been getting pretty liberal lately with cross-listing my stories as "original work." I posted some actual original work (no connection to any fandom) and those got a good response, surprisingly. Then I got to thinking about some of my other stories that I had already posted as fanfic connected to specific fandoms, and how in many ways some of them are actually quite original. I'm still leaving them listed as the fandom that inspired them, but also adding "original work" to the listing to indicate that I'm not taking the fandom so literally.
As I went through my stories deciding what to cross-list this way, I basically asked myself if this story could be done with the characters named something else, and not make people think I was ripping off Fandom. Another way to look at it would be, could someone who's not familiar with Fandom, read this story and get what's going on, without feeling like there's a big backstory they're missing out on?
So, to use a (non-existent) example I used earlier--Star Trek: Enterprise characters, with those names and personalities, but modern-day Earth, members of an extended family, and secretly wizards. Lots of original things done to them. However, to me the web of relationships clearly owes a huge debt to Star Trek: Enterprise, so I would just leave the stories with that listing.
But take another series of stories: Red and Liz from Blacklist as alchemist and junior assistant in medieval England. I'm still going to put Blacklist on it, because I think fans of the show might like to see it and imagine them this way, and there are definitely nods to the show. But really, you could change the names, and be left with an original story about a cagey mentor and his impatient assistant, so I'm also going to list it as "original work."
Obviously it's a very subjective thing and depends a lot on the specific story. I think it's much more a continuum than discrete boxes. My goal is to communicate to potential readers more clearly what the story is going to be like--if you're expecting something very much in line with canon, you're not going to get it; or conversely, if you don't know much about the show, never fear, you don't have to with this story. Thoughts?