Thanks for the suggestion, lexus. I think it makes sense to keep the forum organization as it is, for a few reasons:
We've avoided favoring particular events or topics in Community Discussion (sports, cars, etc.) because they aren't the focus of the site.
We'd rather not delete threads just because an event is over, because nobody likes to see their posts disappear and some discussions are interesting to read even after the fact.
Having a few long threads shouldn't really be a problem for real-time discussions. We have threads for TV shows that have that same type of long-running conversation, and the Tour de France threads each year, and the occasional Olympics threads, have been popular and successful without their own subforums.
The flipside, which I assume is part of what led you to suggest this, is that with a specific subforum there could be many World Cup threads going at once, about different aspects of the games, without them becoming lost among other Community Discussion threads.
That's my first reaction, but mine isn't the only opinion that maters, so others with thoughts about this should speak up too.