This paper focuses primarily on Sandia National Laboratories' Advanced Simulation and Computing (ASC) program progress and accomplishments to support transformative co-design, but also includes the collaborative efforts with Los Alamos National Laboratory (LANL) and Lawrence Livermore National Laboratory (LLNL). The goal of transformative co-design is to develop future hardware and system architecture designs that ease the burden on ASC application, algorithm and system software developers. We describe the impact of Sandia's architectural simulation framework, the Structural Simulation Toolkit (SST), presenting case studies for co-design analysis that include: analysis of multi-level DRAM memory in the ASC Trinity Cray XC40 platform, modern interconnect scaling and photonic networks. The SST analysis capability has also been a useful vehicle for vendor collaborations and has been used to provide concrete recommendations for how optical device and interconnection network designs can address HPC needs. We describe the deployment and impact of our ASC advanced architecture test beds. We also describe our development of a new computational fluid dynamics (CFD) application, and the associated Kokkos programming model abstractions to achieve performance portability on diverse advanced architectures. Finally, we describe our most recent efforts in performance analysis for next generation architectures.