The rapid integration of generative AI into education demands a reconceptualisation of postdigital literacies, moving beyond technical competence to address emerging epistemic and anticipatory challenges. Large language models (LLMs) tend to reproduce dominant educational tropes and generate outputs that are linguistically plausible but not necessarily well-founded, blurring the distinction between coherence and knowledge. Simultaneously, educators are navigating a context of "polycrisis" – ecological, geopolitical, and technological disruption – in which predetermined futures are increasingly uncertain. This context calls for an integrated approach to literacy that combines AI literacy with epistemic and futures’ literacies. In this article, we propose human-AI speculative design fiction as a reflexive instrument for cultivating these entangled literacies among pre-service and in-service educators. The proposed design fictions – short narratives about possible futures – become particularly powerful when co-created with AI, serving as a mirror to examine how human and machine "cognitive assemblages" construct what is perceived as plausible, desirable, or unthinkable futures. We articulate speculative design fiction as a structured literacy practice and introduce the design fictionist approach as a postdigital futuring tool. Through this approach, educators can develop the integrative literacies needed to critically engage and navigate AI-saturated educational futures.