Developing a game concept to tackle complex problems is a challenging process. It requires the collaboration of experts from different fields, from game design to technological development, and from content matter experts to pedagogical knowledge holders. Many books and articles have been written about the design of Simulation Games – from Richard Duke’s seminal 40-steps game design process to recent approaches from 1981 to Freese and Lukosch’s Funnel of Game Design in 2023, and many in between. All these approaches have carefully reviewed related work and analysed game design processes to learn from. As game researchers and developers, we have faced some challenges in applying the existing models when working with content-matter experts without any knowledge of game design. We have therefore developed a new tool, based on the Triadic Game Design Philosophy [1], which we used for participatory game design. We call this tool a GameStorm and describe how we developed it and what we have learned so far.