The first person makes a wish and the next poster plays the evil Genie and corrupts the wish!
Example:
Hermione: I wish I could have a good novel to read.
Harry: Wish granted, but the novel ends with a maddening cliffhanger and the author dies of car crash before he can
finish the sequel. I wish I could fly without my broom.
Ron: Wish granted, but now you're a mosquito. I wish...
and so on and so forth. Get it? So I'll start.
I wish I could eat all I want without getting fat.