All Futures at Once: Supporting Speculative Design for Placemaking with Multi-Agent Social Simulation

Placemaking transforms physical spaces into socially meaningful places, with long-term impacts depending on how future communities inhabit and interact with them. Speculative design helps envision such futures, yet existing approaches often produce static representations that emphasize spatial form over evolving activity. We present ParaScape, a design support system that facilitates speculative design for placemaking by generating dynamic speculative objects through an underlying LLM-based multi-agent social simulation framework. The framework models heterogeneous agents with group-specific preferences and sensitivities, simulating context-sensitive behaviors and interactions that produce evolving scenarios. These scenarios are visualized as image sequences, where each scenario depicts multiple activities unfolding within a place at a given moment. ParaScape builds on this framework to allow designers to explore scenarios, analyze activity diversity and evolvability, and reflect on trade-offs among stakeholder needs. Evaluations through two experiments, a user study, and two case studies show that ParaScape supports critical reasoning and inclusive placemaking.