I have always wanted to create a game where the sole objective is to kill people, in as many fun and interesting ways as possible. Sort of like a combination of GTA and Postal times 100, with no other goals aside from killing. You would be able to pick any major city, and then just go in and start killing. On the streets, in stores, in the mall, in cars, however you see fit. You would have access to every kind of weapon imaginable, from guns and bazookas to nuclear warheads that could level the entire city in a single blast. Or you could use chemical weapons. Anything you can imagine. You could never die or be injured in any way, even if you detonated a grenade in your own hand.
Of course, there would be special unlockable weapons, such as "Unicron" or "Japanese Hornets" or "Oozaru mode", and things like that.
It would be a blast. Or at least, I'd have a blast playing it.
Jam it back in, in the dark.