Most discussion happens in the "General discussion" forum either way, the area gets the most dev traction and therefore gets the most posts.

(Wether they are "General discussion" or not)

No one wants to post their idea for a new battle system in the Battle System forums because it has far less chances of being:

-Read
-Dev response

Personally I like it this way, I hate having to browse through 8 different forums to find discussion on a variaty of topics, and we are not active enough to require splitting the forums up so much. Unless a new patch comes out or someone is on a account suicide run most posts can stay on the front page for a fairly long time, get good views, posts, likes, and feedback.

When the game relaunches and if the community picks up by leaps and bounds then the sub-forums will have a bit more use as the overflow of topics in GD wont be able to keep up. But as it stands the sub-forums serve as nothing more than a un-used overflow for when General discussion gets out of hand in the future.