This thread is unoriginal.
But, I have game ideas going all the time, so here's a more recent one.
A game where you have to try to survive train-wrecks. Of course, the in-game train-wrecks would be much slower than in real life train-wrecks, but not slow-mo. They would work like movie train-wrecks. They can look fast-paced, but still take ten minutes to go down completely. I think that would be really cool (especially if the game could take some randomized terrain then logically construct a train path through it), but wouldn't sell at all. Also, you get more points for the number of people you save.
A game with realistic mechanics (die in a couple shots), but whose point is literally to kill everyone you see. The game would (of course, it's my idea) randomly generate a city, put a shit-load of people in it, make some of 'em cops, give some others guns, then put the player in it with a pocket-knife. So you have to go find a person with a gun, or find a gun in someone's home, then take them out with it, then try to take out the entire city using what weapons you can find. Oh yeah, and your score only gets onto the high-score list if you never save your game. It would focus on realistic mechanics, but extremely stylized action components, like exaggeratedly flamboyant explosions, and exaggeratedly flamboyant explosions.