something like that (subcategories) was done for v5 (was a huge modification, with something like 4 pages of instructions) and it really is standard in 6+, right now we have:
forum summary ==>> categories ==>> forums ==>> topics
are you asking for another level? I am sure it can be done, but would be an extreme amount of work and would effectively lock you into that version, since you couldn't upgrade without doing it all over again...
