everylastbird replied to your post “1o3o1 replied to your post “A small bit of my soul dies with every...”
If you're writing a fusion, you really shouldn't tag it with the fandom you're basing your fic off of, imo. After all, all you're doing is lifting the plot. The characters aren't involved. If a crossover fic has the characters of the series in the fic and they play a significant role, then I can see including that fandom tag
For example, plenty of HP crossovers don't have HP characters, and if they do, it's usually teacher mentions. People looking for HP fic wouldn't appreciate reading that expecting some focus on those characters. But sometimes crossover fics have like...voldemort and Zarkon teaming up, and voltron and Harry have to work together to save the universe, or what have you. That should have both tags, as everyone is getting focus.
I mentioned AUs because I know that that is how some people either keep their stuff organized or use the tag as a way to try to get their stuff out to more people. It’s not something I personally do because, like you said with Harry Potter, I feel like it would be annoying to try and look for content and have a portion of it be completely irrelevant.