Engineer-to-Order (ETO) shipyards face persistent challenges in coordinating complex, high-variation production processes with late-stage customization and dynamic resource demands. Traditional planning tools often fall short in delivering the responsiveness, flexibility and transparency required for effective decision-making. This paper proposes a conceptual architecture for a digital twin system to support production planning and coordination in ETO shipbuilding. Grounded in a case study of a leading Norwegian shipyard, the system bridges high-level planning and real-time operational visibility through integration of sensor data, planning inputs, and 3D contextualization within a modular, cloud-based architecture. A set of four use cases, including spatial awareness and condition-based alerts, demonstrates how the system can enhance situational awareness, detect delays early, and support scenario-based planning. While full twin functionality is not yet realized, a functional digital shadow has been developed and validated with live sensor data. The design emphasizes transparency, scalability, and human-centric coordination, reflecting principles from both Industry 4.0 and 5.0. Rather than presenting implementation results, this paper shares outcomes from the initial design phase, including architectural alternatives and a hybrid integration strategy. The contribution lies in demonstrating how digital twins can be tailored to ETO shipyards, providing both a conceptual foundation and a roadmap for development.