Oh, gosh. So many possibilities.
I do like the idea of going into various universes and leaving their Rule 34 pictures for them to discover. I'd be like some kind of creepy porn fairy! Instead of teeth, I take your childhood, and instead of money, I leave shame and regret.
I think it would be interesting to go back to the Doom series, point at the Doomguy, and tell all of the other marines, "This is the only guy who's going to live. All of the rest of you are gonna end up as corpses or, if you're really unlucky, offset textures. And since most of you die in the rooms with the bad guys who can be killed with the pistol and fists, I'm assuming you all suck." Would they all turn on the main Marine? Would they give up the fight altogether? Or would they, knowing that they are certain to die, continue onward, "Charge of the Light Brigade"-style?