Make a shooter with multiple endings. All the endings are bad except for the pacifist run ending.
Fewer than a certain number of shots fired, not kills, will get you the good ending. Just to make it better, make the enemies into complete bastards so players can't resist killing them. Seriously bad, like they make Nazis look like the Red Cross.
Plus, the enemy force reacts to your violence appropriately, sending exponentially more force at you when you play more violently. If you kill or wound enemies, they start sending more and better troops. If you show that you are a big threat, they scale up the attack to the point where you have no option but to run and hide because fighting will get you killed easily.
To lower the difficulty, you would have to fake your death, but you can only do that a couple of times.
I started typing this as a way to troll gamers, but I kept thinking about the idea until it was something I would definitely preorder. The main character would probably be some kind of spy so a non-lethal route would be viable, and you could MacGyver up some weapons for unconventional warfare.
Tl,Dr: The idea started as a counter-intuitive fps and turned into something that would be awesome.(Partly inspired by Burn Notice and Deus Ex: HR)You're a spy, enemies are complete monsters, fighting boosts the difficulty, faking your death lowers it, only pacifist runs get the good ending, emphasis on makeshift weapons and unconventional tactics when you have to fight/escape.