I think a cool expansion on the current ability to 'collapse' categories, would be the ability to customize your board-index view. Allowing members the ability to completely remove certain categories and forums from what they see.

For instance, on this site, I could choose not to see the 'Sales & Feedback' redirect, the IPSBeyond redirect, the test posting forum and the digg news feed forum. The forums would still be available to me, but they wouldn't be listed when I visited the site. 

Perhaps as an extra though, the ability for the admin to choose certain forums which cannot be hidden, such as news forums, etc.