How about PlanetSide 2? I know it's a fairly old game, but it's free, so pretty much anyone could join. We also wouldn't need to make any servers, since the game has their own.
I used to be in a PS2 clan years ago, and it's a much more fun experience when you've got a group of people hopping into a Galaxy to go blow stuff up than wandering a huge map trying to find fights.
The only issue I can see is getting everyone grouped in a single server, but that's pretty much how all our other servers work.
All we'd have to do is make an [xG] Outfit and just keep people interested in playing by making population type events to get a Platoon and Squads going and go blow stuff up.