Ok, I have an idea.
What you do is you type up a story with Halo characters, weapons, gunfights, etc.
You can make two different ones:
1. A story with an ending.
2. A story where you start the story, but don't finish it, like a short story. But then, some one else comes in and tells the story from where the other person stopped.
To put it simple, lets use two random names, Zack & Dan. Zack starts the story, when he wants to stop, he types "FINISH THE REST", and Dan types "continued from Zack", then types the next part in the story and so on & so forth.
Please do not try to tell the story like it goes Zack starts, then Dan, then someone else, then another person tries to tell the story where Dan left off after someone already did that part.
Then start writing/typing!!!!