I think the idea of splitting it into broad categories like Straight, Gay, Bisexual, Authoritarian, Cuckold, SPH, etc would be great.
Tags would also be great. The way to keep this from getting out of hand would be to limit the number of tags on a story (to say 5).
Also, "title tags" just after the story heading (required) would be helpful: like MF, MM, FF, MFM, FMF; Auth(oritarian); SNM; CBT; etc. Frequently the title doesn't give a good idea of what the story is about. This could be also helped by providing a line of description below the title, such as:
"The Plumber Fixes Mrs. Jones Pipes" (MF, Size, Cuck)
Mrs Jones has an adventure with the plumber's big pipe; Mr. Jones comes home unexpectedly and watches.
"The High School Jock Loses More than the Game" (MM, Size Compare, SPH)
The hunky quarterback is pantsed during a game by his archival.
I've always thought "Fictitious Stories" is dreadful. Why not either just "Fiction" or "Stories"? "Fiction" would be great if you want to limit it to that. However, I prefer just "Stories" because some "real life" stories can be really hot, also. In fact, many "fictitious stories" start with the line, "This is a real story. . . "
Just in passing: Do users know they can already search stories by words in the story or title? I've found this helpful to screen for things I'm interested in.