The characteristics of emerging fields, including blurred boundaries, temporary regulatory absence, and dynamically evolving member roles and interaction patterns, challenge the established value co-creation logic of platform ecosystems, which has been constructed on the premise of well-defined value propositions. The question of how platform firms can facilitate value co-creation among ecosystem participants under emerging field conditions has thus become a pressing concern for both industrial practice and theoretical inquiry. Through a comparative case analysis of two focal firms, Wedoctor and CreditTech, this study identifies two predominant value co-creation dilemmas that emerge within platform ecosystems under emerging field conditions: Ambiguity in co-creation objectives and ambiguity in co-creation modes. Building on this, the study further distills two strategic pathways through which platform firms can activate value co-creation among ecosystem participants. This paper argues that a platform firm’s identity positioning constitutes the core driver of its strategic actions. On this basis, a dynamic mechanism model of value co-creation in platform ecosystems, structured around the triadic logic of co-creation dilemmas, identity positioning, and strategic choice, is constructed. The findings contribute to the growing body of research on value co-creation in platform ecosystems.