Many clean-slate approaches for Future Internet architectures aim at replacing the current architecture with just a single new architecture. Network virtualization, however, paves the way for the concurrent support of multiple network architectures. With today's diversity of networked applications, supporting many specialized architectures is a more future-proof solution. Therefore, we propose a flexible framework for the design and development of new architectures and their concurrent operation. In addition, we provide means for assessing competing protocol implementations and for selecting the most suited one for a particular communication task, based on the requirements imposed by applications and users.