This document outlines an approach to speculative game design called "design fiction". It discusses using games to explore plausible futures and alternative presents by creating diegetic prototypes within fictional story worlds. This allows games to suspend disbelief about change and open up discussions, rather than show how things will be. The document also discusses how speculative game design could embrace plurality, encompass both narrative and simulation, be iterative, participatory, and use rhetoric rather than persuasion. The goal is to help designers reflect on responsible uses of exploring speculative futures and alternative scenarios through game design.