One of my favorite forum games, in Corrupt a Wish, the goal is to grant the previous posters wish, but use their lack of details to make it something bad.
So I will go first. I wish I could fly!
Example:
Post 1: I wish to be a millionare!
Poster 2: Your wish is granted, but the money was taken from the mafia and now they have put a bounty on your head.
So I will go first. I wish I could fly!