A simple game, you grant the previous poster's wish but corrupt it ... then make your own wish.
Example:
Wish: I wish I could fly
Reply: You can fly, but whenever you do you bark like a dog and chase birds.
I wish I had more money than Bill Gates
Reply: You have more money than Bill Gates but it is all in 1p coins.
I wish ...
You get the idea?
I wish I could play the guitar ...
Example:
Wish: I wish I could fly
Reply: You can fly, but whenever you do you bark like a dog and chase birds.
I wish I had more money than Bill Gates
Reply: You have more money than Bill Gates but it is all in 1p coins.
I wish ...
You get the idea?

I wish I could play the guitar ...