I haven't seen any forum games around here and figure maybe there should be some!

This is one that I like to call Corrupt the wish!

Basically you describe how persons wish above could be corrupted.

For example

Wish: I wish to fly.

Corruption: You can fly, but only momentarily as you fly for about 20 minutes you decide to see how high you can go, however you lose this power about 5 stories in the air and fall to your death.

Easy and fun! Just make sure that your wish can be corrupted.