I always wanted to see/make a game called Evolution, where you start off with an incredibly simplistic game in which doing well earns you points. These points do not upgrade your avatar or game entity, however. You upgrade the game. So you can purchase things like extra colours, more visual detail, at some point upgrade your camera angle to isometric or full 3D. The real crux of the idea rests on being able to upgrade the gameplay- adding more player units can evolve the game into a tactical shooter, or you can change the method of competition and end up with a sports or racing game.
In some ways, the game could naturally document the evolution of games historically (although in a more symbolic way with the later levels). In other ways, it'd be interesting to consider the possible implications of certain combinations- perhaps discover new game genres in the process.
