This Design Thinking Research Project is based on the insight that Design Thinking is at its best if tangible prototypes can be used to envision new products and services. By using prototypes that can be experienced and judged, assumptions, needs, and insights can be validated. Also, a common understanding is established between the Design Thinkers and all end users. However, for complex software systems with multiple users usually such tangible prototypes are not feasible. To overcome this problem, we propose a scenario-based prototyping approach for designing complex software systems that is based on models, both structural and behavioral models. The approach will support the stepwise and interactive enrichment of the prototype model, the traceability between the artefacts collected during earlier design phases and the scenarios. The models provide a more formal outcome of the design process for the downstream engineering activities, so that the gap between design and engineering can be narrowed.