Riot,
Sorry for snapping -- I just get tired of having to make these points very frequently, but maybe you weren't involved last time. See, e.g., the recent moderation and speech thread started by the mods. The brief version is that this forum enjoys deeper conversation on a wider array of topics, and stronger relationships, than almost any other forum I have been on; and I think its position on free speech is strongly implicated in this, given that on many forums, quick claims of offense end any topic once that has become an endorsed tactic. There's more, but you can go find the past threads if interested.
My point is, I don't think anything can necessarily attract more users. Let me spell it out again:
i) While there were a lot of people who had played diplomacy for years and had no site, growth was fast, because those people were getting on and finding this site.
ii) That process is over now, as you say. Thus, new users are people who hear about diplomacy anew, or are introduced to it. This is an intrinsically slower process, because we don't get the benefit of a 40-year backlog all joining at once.
iii) Thus, the only way to speed it up more is to introduce more people to the board game Diplomacy (or the online version I suppose) who haven't played it before at all, and that is pretty slow. It's almost always word of mouth, and while I suppose we could advertise, I'm not sure there's a big point. Encouraging use among high school civics teachers might also help, though I imagine the mods would hate us.