That's all you can do, I think.
You can use forum catogories though. So for example I've got a forum category called "Raiding" that people can post in for conversations generally about raiding, but then there are sub-forums of "boss tactics" and "guild raids".
It may just be a matter of coming at it from a different angle.
Sub-sub forums would be a super handy feature, I think, but it looks like you just need to be a little creative with your forum/sub-forum set up for now.