Adaptive gamification is an emerging and fast-growing research stream, that enhances traditional gamification approaches with user-centered, personalized and adaptive incentive mechanisms, tailored to a specific characteristic of different users and contexts. While game-like elements have been successfully applied to increase end-user engagement, satisfaction and task performance in different domains, the effectiveness has often been mixed, highly context specific and varied among individuals. In order to understand how adaptive gamification approaches can be developed that overcome such problems, we have conducted a systematic literature review that identifies main issues and challenges in current literature on adaptive gamification. The analysis corpus is composed of 43 identified studies and includes supporting theoretical contributions from related research areas. The performed analysis provides several contributions. First, a conceptual matrix of adaptive gamification design is proposed that identifies major dimensions of current approaches and classifies these accordingly. Second, we came up with a thematic overview where the identified literature and their related studies are assigned to the designated areas. Finally, we identify five research challenges and propose a research agenda that can serve as a basis for future research directions and for practitioners who want to apply adaptive gamification strategies in real-world applications.