Inspiration: Angry Birds

The object of Angry Birds is to smash the buildings and the pigs. At the end of the level, assuming you got every last pig, you get a star rating. As you earn stars through the game, new levels are unlocked.

I'm going to try this with my classes. Each assignment will have a star value. Some only take a day, they earn a student one star. Some take three days or more, these can potentially earn a student three stars. Like in Angry Birds, the quality with which they complete the larger, two- or three-star assignments, will determine how many stars they actually earn. Students' final star count determines most of their grade. There would be a variety of assignments available, so students would be able to choose their own breadth and depth depending on what interests them or what they're skilled in.

Another feature I'd incorporate is the unlocking of new levels by earning enough stars. Ideally students would  be motivated by certain 'levels' or assignments so they would earn the necessary stars beforehand.