It certainly is an interesting question.
I actually started thinking about this yesterday when this question was first posed. But, what if we had a bunch of the heads of the forums come together and do a little "United Forums of America" forum or something!
One of those 'super' forums with multiple topics and each category could be moderated by the expert of their 'field'. For instance, if we have a travel and leisure section, I could watch that area since I have a lot of 'vacation' experience. We could have a political section (argue crazy) and another section about Fashion (petite chic) and sports (talkphillysports). Plus, we can have one forum for 'local issues' with certain states represented New York, Pennsylvania, Delaware, etc.
And with the SEO knowledge that Dave and Brandon and some others have...wouldn't that be a huge forum?