This article reports on the results of the 12th ACM/IEEE International Workshop on Software Engineering for Systems-of-Systems and Software Ecosystems (SESoS 2024) in which researchers and practitioners discussed ideas and experiences on the research and practice for the development and evolution of complex softwareintensive systems, more specifically systems-of-systems (SoS) and software ecosystems (SECO). SESoS 2024 was co-located with the 46th IEEE/ACM International Conference on Software Engineering (ICSE 2024). After a decade running this workshop, the SESoS community is advancing on how to cope with the different dimensions that should be considered in the engineering of those classes of systems (i.e. technological, organizational, and social), and also is taking awareness of newer challenges for inclusiveness and sustainability. In addition, benchmarks for conducting research on the areas as well as approaches for investigating emerging domains (smart ecosystems) and non-functional requirements on those systems were also pointed out as relevant challenges.