You're on to something.
Getting this done could be very easy on the user-end of things... The control panel would be a different matter.
But then you face a distinct problem - which category is the forum listed under in the breadcrumbs at the top? Remember, the page is cached....
One idea would be removing the category breadcrumb altogether.
(This was one of the issues when subforums and the like were discussed a while ago. The whole idea of subforums is very easy, and like this, easy enough to code in... but it's hard to manage - the cpanel would be a nightmare...)
