The service-oriented paradigm typically separates the development of systems into service engineering and application engineering. In our opinion, these two disciplines cannot be completely separated, as otherwise, there is no assurance that services provided by service engineering can actually be used by application engineering. In this paper, we describe service-oriented requirements engineering (SORE) as an appropriate means for mediating between the two disciplines, and we identify the provision of service capabilities on the right level of abstraction and variability as the most crucial aspects SORE has to assure. For this purpose, we present the adoption of product line technology in service-oriented architecture and its closer alignment with business process management as a promising approach, and list related principles as well as challenges we plan to address in our ongoing research.