This particular forum is very active given the number of visitors we have. Threads are divided two major ways at the moment; theorycraft and general information. I am going to split the forums along those lines. There may be a later split to include TPS, etc.
As a general rule, I do not like creating new forums unless they will be active or have a particular value. I'm not a big fan of sites that have 20 different forums with only 3 that are actually visited.
Actually, I think that's a pretty good idea, and I don't think you'll have to worry about inactivity with either. I think that what makes this forum different from the WoW warrior forums is that people come here wanting to learn more about theorycraft and improving their skill at tanking, not just to talk about how fun being a warrior is all day. :P