Hmm, we used to have a riddle thread going awhile back. The way it worked was just that once someone answered, they waited for confirmation that they were correct then posted their own riddle. That seemed to work out ok, although went quite a bit slower. In the event that someone didn't respond with their own riddle then the previous poster would just post one, or if they didn't get confirmation for a couple days then they would just assume they were right and went ahead.