What IP said but make them forums, not sub-forums.
Personally, I hate the idea of sub-forums besides saving space, and that's all it does. Sub-forums means that the forum will generate less traffics because people can't track it as easily and it's much easier to scroll by the link because you aren't looking at it, you look at the last posts.
I hate subforums as well... but as it stands...?
The idea of merging the two discussion fourms in the first place was to make them generate more traffic (Meaning getting rid of Lite and Serious discussion) making ANOTHER fourm would just generate LESS traffic. Sub Fourms can be bad in certain ways, for instance. BeeR (Or whoever it was) said that he couldn't check the "Creative" fourm easily because most of the time the most recent thing was not something he could help with. The chances of having a sig making, movie buff, who's written a book using HTML coding are very low. In the discussion fourm it's all consistent you're always discussing things. In the creative fourm it's wide varity of things which isn't the case of Discussion fourms.
In other words try out sub fourms first and if it's really so horrible switch to normal fourms. Since using sub fourms conserves space anyway there's no point of trying something that will probably work but take up more space than trying something new that could work but uses less space.