I think the tag page only lists the tags that have actually been used, so maybe that's why I didn't see the ultimates one when I checked.

on the misc thing, I think of AU genre as like... Tony is a WWII mechanic.

I'm not advocating for the misc tag, just thinking out loud. I don't suppose anyone would want to search for random universe mashups, and it seems like everyone is good about listing universes (or lack/combination thereof) in their headers anyway.