“original work- alternate universe” seems to be being used for when a fan writer creates a whole new universe with their own intricate lore. Like, for instance, weird Undertale variants.

If your work is deliberately creating a unique universe unto itself, where the elements from the original are more of a vehicle to tell your own intricate story, I’d use that tag.

If you’re just writing a fic without bothering to dig too deep into what’s canon or not, I’d use “AU canon divergence” or a flavor of fanfic au tag that suits it.

If anyone else wants to weigh in on this please do!