Ok, I think this has been discussed in the past, but I'm not sure where it went, or what ever happened to the idea. Basically the way this forum works (to my understanding) you have a tree structure like the following:

Discussion Area
-Categories
--Forums
---Topics
----Posts

My proposed Idea is to do something similar to what vBulletin does with thier Sub-Forum idea. Basically a sub-forum inside a forum, and then you can have more sub-forums inside of sub-forums, however deep you want. I have come really need something like this on my website, and have been thinking about it for quite some time.
The result would be something like the following:

Discussion Area
- Category
-- Forum
--- Topic
---- Post --- SubForum
---- Topic
------ Post ---- SubForum
----- Topic
------- Post
Notice how SubForum sits inside of Forum and the green tree(another subforum) sits inside the first SubForum which is in the Forum. I'm not sure if anyone has a mod already, or whats going on, but if not I'm going to start working on/organizing such a mod because I think it would be most helpfull for me, and many others.
Basically all I think a person would need to do is create another column in the FORUM_FORUMS table (I think that's the one) which would specify a parent forum (Its ID). All forums falling directly under the main categories would have no value, so they would be "root" forums. Let me know what you think. <