This is an idea I had called "Story Game of Constant Improvement".
1. The first post contains a story with 3-6 sentences, including setting, genre and any additional "rules", if needed. The plot, per se, is defined loosely by the beginning setup.
2. Each poster quotes the original messages and responds to the last post in the list doing one of these things:
(a) Adding a sentence between two other sentences,
(b) Insert a new sentence between existing sentences,
(c) Replacing one sentence with two sentences that either add to the story or change some aspect.
3. This continues with the next poster doing the same.
4. The first poster that notices that the word count is 500 words or more declares the story done and can begin another one using the same rules.
5. Characters, places, etc, can be introduced, as long as the story remains coherent and consistent. For instance, do not add something that causes a conflict in the story, such as a man suddenly changing to a woman, etc.
6. The Title counts as one line that can be changed.
7. All the original lines, including the Title, must be affected before going back and changing other peoples new lines.
8. Mark new lines in green and the removed lines in red, if you like. When all lines have been changed, the next poster removes all the red lines and strikeouts to clean up the story, before posting. Any poster may correct typos as you go.
9. Keep the overall story correct and consistent. Each sentence should stay on a separate line to simplify editing, but blank lines can be kept between paragraphs. The entire story should be quoted each time, to show the edits that have been done.
I will start and then give an example. I think it will be fun. I hope you enjoy it.