In this talk I will consider a design manifesto for using games as speculative designs through which players might explore scenarios that represent plausible alternative presents and speculative futures. The talk will review future orientated design techniques such as Design Fiction and Critical Design, to create a new manifesto for Speculative Design that is informed by other game design approaches, such as Critical Play, Persuasive Games, and Procedural Rhetoric. The manifesto aims to provide a design frame for desirable and productive future practice when creating games that facilitate discussions around complex societal challenges.