Building on the foundations set by the AP Computer Science (CS) Principles course, this project seeks to dramatically expand access, especially for high school girls, to the most exciting and emerging frontiers of computing, such as distributed computation, the internet of things (IoT), cybersecurity, and machine learning, as well as other 21st century skills required to productively leverage computational methods and tools in virtually every profession. We present the design of a new, modular, open-access curriculum called Computer Science Frontiers (CSF) that provides an engaging introduction to these advanced topics in high school (that are currently accessible only to CS majors in college). The project leverages NetsBlox, a powerful yet easy-to-use visual programming environment that has been shown to increase engagement and interest in computing. Additionally, NetsBlox supports effective collaboration while facilitating learning of distributed computing, networking, and cybersecurity in informal settings.