Cyber-Physical Systems (CPS) are complex physical systems interacting with a considerable number of distributed computing elements for monitoring, control and management. They are currently becoming larger as Cyber-Physical Systems of Systems (CPSoS), since many industrial companies are transitioning their complex systems of systems to software-intensive solutions in different domains such as production or manufacturing. Following the development and dissemination of DevOps approaches in the Software Engineering world, we propose the Twin-Driven Engineering (TDE) paradigm as a way to upgrade the role of Digital Twins (DT) to become a central point in all the engineering activities on the CPSoS, from design to decommissioning. Since CPSoS can be highly heterogeneous, we rather target the support for producing and maintaining a single integrated virtual representation of the CPSoS (i.e. a System of Twins) on which it is possible to perform global reasoning, analysis and verification. However, such a new paradigm comes with several open research challenges. We provide an overview of the state-of-the-art in key areas related to TDE. We identify under-investigated problems in related work and outline corresponding research directions.