My bet is: "Bitter wishes". Vague enough to leave ample freedom to the partecipants, and with that feelings of simplicity and complexity at the same time, like a game jam theme.
Here some quick ideas using this theme:
- Everytime you complete a level, you will be granted one wish (powerup), but with a drawback [Top-Down-Shooter/Strategy]
- You have to give some nasty presents to your hated coworker(s) without him finding out [Stealth/RPG]
- Maybe your protagonist simply whishes to eat some dark choccolate [Platformer/Puzzle/Exploration]