Posted this in another thread, but classic/vintage/arcade/video game keys seem like an easy idea. There are Pac-Man keys in "ZOMG Imsto Dyesub Group Buy" but that leaves Donkey Kong, Galaga, Asteroids, Space Invaders, Tempest, Asteroids, Frogger, Dig Dug, Joust, Qbert, Centipede etc.
Seems like there would be lots of simple graphics to use since the technology restricted game elements to small simple abstract things.