I think that this would be brilliant, and ties together the existing functionality nicely.
At present, I manually force users (via the Rails console) of certain groups to have subcategories of interest to them in their sidebar - otherwise they simply can’t find them unless they have a notification.
I’m less sure of this, as it becomes tricky when people have more than one interest - this has happened to us as our site has grown, and has limited its utility significantly.