I was thinking of LJ comms across fandoms and the idea of your buffyverse comm sounds very similar to the one behind where_no_woman, dedicated to the women in Star Trek, which seems to be doing fairly well for itself, so maybe it could be used as some kind of inspirational model? Also, talking about pretty active LJ communities, be_compromised, dedicated to Clint/Natasha from Marvel's Avengers seems to me one of the most active comms I participated to, lately. The key is probably the extraordinary team of mods who constantly propose new and interesting activity, though being part of a new and large fandom probably doesn't hurt lol...