That's kind of what I'm planning with factions/groups and such, but we can definitely give this a shot!
The main reason why I'm not restricting each worlds to one thread only is because I want this to be open to everyone. I know that things will get cluttered (depending on how many people join i guess lulz; if a lot of people join, then that's going to be a completely different story), but I don't want anyone to look at a thread, see that it's already been going on for a while, and feel too intimidated to join; the thread creators might not keep the thread updated with a summary either, which could keep new people from joining. I want this to be open to everyone, so that's why I'm worrying about it.
But we can still give it a shot, at least initially. If it works, then we can keep it around, but if not, then I'll let there be an unlimited number of threads for each world.
I'm also tossing around the idea of closing the current threads whenever a new event starts, since the event roleplays will create changes in the overall story, but that's completely up to everyone. I may do a "test round," see how it goes, and then, if it works well, then we'll do it again in the future. But, if it doesn't, then we won't worry about it.