After discussion, we're going to go with SciFi/Fantasy this year, because we think it has broader applications. I do think combining that tag with Genre Shift will work, as it increases your chance of matching on someone who's interested in AUs and is interested in SciFi/Fantasy.

Keep in mind that you'll still need the optional details to request anything specific, and people requesting (for example) historical AUs will be in the same boat.

Thank you the feedback, though!