Deciding what kind of smart cities do we, as a society, want is not only a political question but also a matter of envisioning possible futures. The speculative narratives that designers produce to support their imagination are called "Design Fictions." We share SUBMERGED, a cross-platform project that combines game design, interactive narrative, and urban exploration with the objective of empowering citizens to produce their design fictions. Following a "Research through Design" practice, we describe our process for creating SUBMERGED, we synthesize some critical insights from our experience, and we urgently call for a dialogue between semioticians and design researchers on these topics.