You might as well have a 'general' forum and have categories for "Sports", "Funny Stuff", etc.
Then, when you see a particular category taking off and becoming very popular, then you can create sub-forums and gauge the interest in each.
After some time (a year at least) that you see that activity and interest is still high on those particular aspects, then you can re-theme the same site and reorder the categories to feature those most popular forums.
Or, you can even take that very popular portion of your site and create a new site based on just that topic.
It all depends on what you want to do, what you can do, and how much patience you have. Running forums can be VERY frustrating and WILL be very time consuming. It ain't some 'set it and forget it' sort of website.