Well, that's strange -- but we do have some stories that are Action-only, and certainly wouldn't belong in Adventure, so I'd still lean towards keeping them separate. Plus many people would likely just say Adventure when submitting a fic from FF.net, and it'd be a hassle to have to fix them all to Action/Adventure.
I'm banking on people just copying the genres from the fic they're submitting. Most fics are coming from fic sites, like FF.net or HPFF, which have genre descriptions already in place. The only place I can think of where a story might not have a genre given is, like, Livejournal or something. Typically a fic with no genres would just go into the General category, since we don't want people to just make stuff up unless it's super obvious.
We could always just make notes in Category:Genre: Mystery and Category:Genre: Suspense describing the difference between the two... Mystery is like a "whodunnit" where the point is to solve a mystery, and Suspense is more like "thriller," where the reader has all the information but are kept on edge by "will they catch them? will they survive?"
I guess we can just wait on Supernatural and Sci-Fi to see whether many stories come in which are described as such. I don't really read much of either, so I don't know if the categories would be wanted...
I guess I just really want to keep it intuitive. What would a person submitting a new fic naturally want to put in the "genre" field? And since that'd typically be the exact genres that the author provided, I think our system should be mostly based on the genres that authors tend to provide -- which are the genres provided as options by fic sites.