This paper addresses how far co-creation methodologies can be applied to policy-making innovation in the platform economy. The driving question is how co-creation collaboration-based policy-making can increase diversity and strengthen the participation of actors. The analysis is based on a three-year case study on the platform economy in Barcelona, describing how co-creation dynamics contributed to the participatory definition of local public policies and agenda. The methodology is based on participatory design techniques, involving participant observation and content analysis. Results indicate that co-creation can increase participation diversity aligning academic, economic, and social viewpoints in policy innovation from a quadruple helix perspective. In addition, collaboration schemes assist in engaging a wide diversity of participants in the policy ideation process which, in this case, resulted in 87 new policy measures, with contributions from more than 300 people of different backgrounds and perspectives. The case study demonstrates the value of a cycle of collaboration going beyond mere symbolic engagement or citizen support to public policy-making. It further shows the importance of combining co-creation with methods of action research, strategic planning and knowledge management, as well as with face-to-face interactions and online channels.