This might pose more significant UI challenges (and may just not be the direction this site wants to go in), but as a fan of tabletop RPGs, it would be awesome to be able to search by the particular system that's in use in a given thread. On the one hand, it's only a single new tag for any given thread, but on the other, it's a pretty complex taxonomy (if done with full fidelity), and could have implications in the search subsystem as well.

(Added as https://github.com/danneu/guild/issues/147, so that you can close it independently of the main Tags revamp question).