Personally I think the idea of multiple forums is rather antiquated to begin with. When you look at modern blogging type software, tags and categories make more sense. It's easier to search and organize however the user wishes to see it. But phpbb is still a bit in the past. I feel there's a lot of room for improvement with this software without making it lose the feel of a forum.
Imagine users being able to sort it how they like and pin their own categories and forums based on their search terms. Of course ultimately such a thing would rely on people properly tagging their topics. :P