Why don't you use existing data from bookmarking sites? I already do this manually--when I'm in the mood for a particular sort of fic, I find a fic in my own Pinboard bookmarks that's similar, go to the global page for that link, click through to a few of the people who saved that link, and see what else they're saving. It would be awesome if there were a service that could just tell me what other fics those people are all saving.