This paper describes the early stages of an experiment investigating the role of the computer as a creative collaborator in the game design process. We introduce the Shibumi set, a closed game system so simple that its rule space can be completely defined, yet deep enough to allow interesting games to emerge. Constraining the search space to such a closed system has computational benefits, but had unexpected effects on the creative process of designers during a related game design contest. These effects yield some insight into the creative process of experienced game designers, in particular, the way they search for rule sets to realise desired behaviours, and suggest a simple unified model of the game design process. We suggest ways in which these insights may be incorporated into future work, to produce software that might not only search for new games more effectively and assist the designer as a creative collaborator, but to automate the game design process in ways that might be perceived as more creative.