WhatsApp, as the world's most popular messaging application, offers significant opportunities for improving the reach and effectiveness of engagement projects. In collaboration with the International Federation of Red Cross and Red Crescent Societies (IFRC) we designed WhatFutures, a collaborative future forecasting engagement for global youth using WhatsApp. WhatFutures was successfully deployed with 487 players across 5 countries (Kenya, Bulgaria, Finland, Australia and Hong Kong) to inform strategic change within the IFRC. Based on our analysis of the activity - including 16,100 messages, 95 multimedia artifacts, and a post-engagement survey - we present a reflection upon the design decisions underpinning WhatFutures and identify how decisions made around group structures, processes and externalization of outputs influenced engagement and data quality. We conclude with the wider implications of our findings for the design of engagements that best utilize the affordances of existing messaging applications.