I've decided upon the twist that'll be enforced upon your entries. When coming up with the twist, I had to think of something that would not hamper your ingenuity, yet would still restrict most if not all previous projects from being submitted.
The twist is this:
Each and every background and sprite can only be used in '4 bit' mode (16 color). This means that you can only have the possibility of 16 colors per bg or sprite, per scanline. Enterprising developers can make use of hblank or other trickery to increase their color count, but the sprites and backgrounds must remain in 4 bit mode throughout the duration of the game. This includes intro screens, splashes, etc.