I'd suggest just disabling the ability to make threads and make one (or two, but I think one might be best) thread each month. At the end of the time period, delete them and post a new one. Or disable posting.
Again, you'll need to advertise, but asking others to link it won't get you many users. And if you plan to host a multiplayer server for some of these, you'll need a way to get lots of people online at once.