Open Source Software (OSS) communities face a wave of uncertainty as Generative AI (GenAI) rapidly transforms how software is created, maintained, and governed. Without clear frameworks, communities risk being overwhelmed by the complexity and ambiguity introduced by GenAI, threatening the collaborative ethos that underpins OSS. To address this gap, we present a Socio-Technical System (STS)-guided conceptual framework that applies McLuhan's Tetrad as an analytic lens to articulate how GenAI reshapes the socio-technical dynamics of OSS development. Through a scenario-based exploration across four components of the STS-guided framework, software practices, documentation, community engagement, and governance, we identify plausible socio-technical impacts and outline a corresponding Roadmap for sustaining OSS communities in the Age of GenAI. This Roadmap will enable OSS researchers and practitioners to interpret emerging transformations, anticipate challenges, and design interventions that foster long-term community resilience. By adopting this framework, OSS leaders and researchers can proactively shape the future of their ecosystems, rather than simply reacting to technological upheaval.