This paper describes the design of a system to enable research, development, and testing of new software stacks and hardware features for future high-end computing systems. Motivating uses include both small-scale research and development on simulated individual nodes of proposed high-performance computing systems, and large scaling studies that emulate a sizeable fraction of a future supercomputing system. The proposed architecture combines system virtualization, architectural simulation, time dilation, and slack simulation to provide scalable emulation of hypothetical systems. Virtualization-based full-system measurement and monitoring tools are also included to aid in using the proposed system for co-design of high-performance computing system software and architectural features for future systems. Finally, this paper provides a description of the implementation strategy and status of the system.