Balancing design challenges and design skills is difficult. There is a need to scaffold game design and to have a well defined space of game projects connected by pedagogical stepping-stones. We primarily use the notion of Flow as a psychological model of motivation and learning. Other motivational approaches, such as motivation and personality, intrinsic vs. extrinsic motivation, self-efficacy, and peer influences will be explored in the context of the project. Based on our experiences, the skills versus challenges space of Flow can index and relate a number of game projects starting with a simple Frogger game all the way up to Sims-like games. The path in the figure denotes a scaffolded learning trajectory, which can be supported by scalable game design. The goal is to remain in the optimal flow zone as much as possible. This can be achieved through various forms of scaffolding, such as explicit just-in-time instruction, social learning support from interactions with other students, or curricula designed in anticipation of the next challenge. Especially from the viewpoint of broadening participation in CS, the lower left point in the Flow space needs to be extremely conservative. We cannot assume that participants have any game design or programming background. Our benchmark is to have 10-year-old elementary school children without programming experience make a simple but playable Frogger-like game in about three hours.