Players in digital game worlds, such as Massively Multiplayer Online Games (MMO), generate long standing histories with their characters, but cannot express or see persisted traces of their adventures within the environment. I argue that understanding the role of persistence and the emergence of player narratives in digital game worlds can lead to novel systems and tools for increasing players'engagement and help game designers produce content. I combine multiple methods for understanding player narratives, and present We Ride, an MMO that embodies and tests the Narrative Substrates framework for building game architectures and mechanics that leverage persistent player narratives. Multiple studies show how Narrative Substrates successfully encourages players to codesign new content from their own persisted narratives, letting them shape the world and contribute novel content. Finally, I discuss the challenges of this work and suggest several promising directions for future work.