I'd suggest some "themed" contest, where you have free choice of the gameplay, but where you should stick to a certain theme for visuals and style.

My personal fave would be an abstract (hope this is the right word) themed contest. Since almost everyone (including me) is dissatisfied by the current lack of innovative gameplay in modern games, a contest that would force you to create some abstract and innovative games would certainly get interesting.

So my vision of the next PGD-contest :
"Create a game with an abstract or innovative (often both are connected) gameplay and and also abstract visuals, using e.g. cel-shading or other NPR-rendering methods."

I think that would make up for some nice entries and isn't too restricting on what games can be entered in the contest.