Ideas I’ve had so far!

1. Something to do with suffering consequences from your actions. Specifically, something where the ONLY way to do anything is to do something currently which will incur some consequence later. So, the past you is your enemy. This could be sort of like an arcade game with some character who, to be upgraded, agrees to be attacked by N enemies in one minute. Then, to survive, one must advance fast enough to handle the enemies, but not so fast as to spawn a deadly wave of enemies.
2. Some sort of traditional game, except you play as the enemy instead. I’m pretty sure this idea has already been had by, say, everyone else, so I’m not going to do it.
3. A sort of “puzzle” where there are mirrors and other objects that make copies of you, reflected and perhaps in a location with some differences. For instance, mirror you might be next to a box of TNT (conveniently), but you might not be, so if you punch it, “KAPLOW!” goes mirror you.
4. A game where one has to fight against a lot of “good” people who think you’re the villain in order to… to do something.

I like the first one the most because it is totally within the scope of a Ludum Dare and uses the theme in gameplay. Maybe I could play with the idea of you being a villain more than just to yourself – maybe you’re in a position of power over others or something. E.g. you have to guard them, but you’re a really crappy guard who also spawned the monsters. I shall get down to business tomorrow after the magic of sleep (or, a variant of sleep I like, waiting in bed, thinking about stuff) has made my mind clearer and chewed over this idea.