I realize this thread is almost 2 years old, however it is still a sticky at the top, thus I am going to post my suggestion here.
So I was trying to think of a more controlled (I.e. less annoying) build a story game and started thinking about Mad Libs. How about a forum wide Mad Libs game?
The numbered grammar list (1. Adjective, 2. Noun, etc.) can be listed and then and each person can post 1 word (preferably in order so it doesn't get confusing.) Example: #1 Ginormous, #2 Asparagus, and so on...
Then once all the words are added, the filled in story can be posted.