It's bad enough having people go into forums, not read any notices or stickies regarding what a forum is for and then post something that shouldn't go in that forum. This suggestion would only make it worse. Also the design isn't very nice and potentially that choose forum could be massive if you have dozens of forums.
However, I'm very sure you created this same suggestion quite a while ago. Your mockup screenshot had the Post New Thread button under each category on the forum list. I can't seem to find the thread, though.