I'm vehemently against fandoms being shoved out of their own tag. Don't make a new tag, the canon tag is where you're supposed to be. You're only making it harder for new and existing fans to find the community they're looking for!
Do you know what the actual etiquette is? The one that's been around since Tumblr began? The one that's been followed dutifully up until the last few years?
Once an AU becomes so prevalent it starts becoming bothersome, YOU DON'T MAINTAG THE CANON FANDOM. Yes, this is a gray area when it comes to what constitutes as bothersome. Yes, bloodymary is WAY beyond that line.
Instead of bailing on the canon tag, SPREAD THIS MESSAGE. If someone refuses to follow this etiquette, BLOCK THEM.
You owe it to yourself, to your community, and to new fans who won't know the fandom exists in some obscure tag. Stand your ground and be vocal about this etiquette.
PS-- to the AU fans who insist they won't be found without the canon tag: yes you will. People subscribe to a tag when they know that's the fandom they want. People use the REGULAR SEARCH function to discover it. If your post has canon character or franchise names in it, or even non-canon tags, a regular search will display those.
Be considerate. Acting like an ass will only make possible fans hate your AU when it's shoved down their throats.