Yes, the trouble is, either you have fairly moderated tags and categories, but you can't add to them (or not easily, without asking the mods), or you have AO3 where you can add them all yourself, but so can everyone else and do it Wrong and Annoyingly.

There was once, for reasons, a "The Avengers - Ambiguous Fandom" tag. People used it, even though it was basically tagging your work "help I don't know whether this is an American Movie/comic book thing or British cult spy thing featuring Emma Peel goddess". (It's been tidied up now, but that is people and tagging for you.)