A few ideas for a Storytelling Event

Hi!

What I am going to propose is nothing new, it is a common entertainment on various forums. Yet, I did not see something like that going on in here.
The idea is that participants would tell an ongoing story, picking it up from the place the previous user left it.
The thing is, without the sense of direction or a structure to follow it would probably unwind into a chaotic mess. I've come up with some ideas that would enhance the experience:

1. As far as I know there are some limits when it comes to the topic length. I don't know is it a restricted number of comments and/or replies, or are comments restricted to a specific number of characters. I'll need to know.
   Still, restrictions are good in this case: It forces the story to find its way toward the end. That way it is possible to delineate a structure for a story beforehand.
   For example, if a topic is restricted to, let's say, 60 comments then we could agree that first 20 comments would serve as an introduction or background for a story, middle 20 comments would serve as a main body of a story and the last 20 comments would need to wrap the story up.

2. I've come up with a nice mechanics that could be used to choose the mood, characters, genre or any required element of a story. I think it would be nice if the theme could be chosen randomly. I wondered if there is a webpage on which i can simulate a die roll so that everyone could see a result.
   Then I thought - wait, I AM on such a webpage! The choice could be made by opening a giveaway which would be entered by a precise number of users. Then, every user that joined the giveaway would be assigned a theme of their own. For example
   Happy - user1
   Sad - user2
   Angry - user3
   Thus, the winner of the giveaway would signify the theme of a story!
   The giveaway could contain two or more copies to win and the winners could signify two or more themes of a story, for example theme X vs. theme Y

3. I also had some thoughts about the order in which the users would take part. My idea is that before the event users would report their wish to join so that the order would be known beforehand. As to that idea it is a little more complicated and I will describe it if there will be anyone willing to participate and after I learn more about the possibilities and restrictions on this forum.

So, please, share your thoughts about this idea!