Is the point to make it a technical challenge or a game design challenge?
For technical challenge:
- All graphics/sprites should be contained in a single sprite sheet of limited size (color shifting allowed)
- No hardware acceleration
- Every asset must be procedural somehow
For game design challenge:
- Very limited screen resolution (think giant pixels in old consoles)
- Very specific control scheme: 1 button, NES gamepad, text parser, guitar controller, dance mat, wii remote...
- Random combination of genres: Roguelike + dancing game, driving + platformer, metroidvania + cooking game, tile matching + endless runner + roguelike (wait... that's 10000000 and You Must Build A Boat
Although some of those sound like the Ludum Dare or similar contests.