The Problem of Unicorn Epicycles

[Aleh]

A good fanfic diverges or expands on the canon in an interesting way that stays true to its (the fanfic's) inherent conceit of the exact expansion/divergence. Changing the ground rules of the story is about as divergent as you can get and as long as the story is interesting, it should not be an issue. Like taking the Harry Potter series and transforming it from a magical school to a school for techno-thriller spies and assassins changes the entire base concept of the world (magic is real) and changes it to James Bond was real.;

Sure. I've actually seen that specific premise done reasonably well (although I didn't get into the fic as much as I should have). The thing is... that's all the fic changed. That sort of premise only really works as a fanfic so long as the story and characters are still recognizable.

Or, to put it another way, "Harry Potter as a spy thriller" only works so long as Harry Potter is recognizable as Harry Potter. The "as a spy thriller" bit becomes your unicorn... and you only get one.

At that point all you're doing is failing to commit to writing original fiction, by using a few names and maybe a few setting mechanics general enough to count more as tropes, from something popular and putting them on your own setting, tour own characters, and your own story. That's just authorial cowardice, and/or the literary equivalent of click-baiting to get people to look at it under the pretense that it's about something they know they like.

My initial thought was that you were replying to batzulger above, but then I realized that you could be talking about Beware of Cloud or any of the assorted hypotheticals we brought up as well.

But, that said, I don't really think there's a firm line between when something works better as a fanfic and when its fanfic elements are holding it down. I just think that making deliberate changes to canon can be a slippery slope from an authorial perspective.