I'm hardly 100% in the know regarding the lore of Warhammer fantasy, but I do know a fair bit. I'm still somewhat miffed that the Skaven aren't going to be in it as a playable race.
They'd be seriously fun to RP as.
An Imperial Militia sounds best to me, so long as it isn't attatched to any large cities. Perhaps a sort of volunteer border watch? Not too elitist, and not too restricting or static, with ample opportunity to explore and room for roleplay, inter-guild events, and pvp. Sounds perfect, and the Militia concept's been tried and tested in UO.
That said.. Henry mentioned a guild of fanatics. Top notch thinkin' there, though perhaps better suited to an established RP community.