The worldbuilding practices of science fiction authors have the potential to play a key role in society, given that they involve the design and depiction of complex, alternative realities set in the future. This potential is acknowledged by Transition Design — an emerging area of practice that melds futures-based narratives, foresight, and systems-thinking, amongst other disciplines. Transition Design goes beyond social innovation to envision radically new images of the future, and pathways towards more sustainable systemic states. To facilitate the design of and transition towards sustainable futures, this Major Research Paper introduces the Seven Foundations of Worldbuilding: a model that integrates a new superstructure of complex systems with backcasting methodology.