That's a tough one. Maybe you could help work on a JIT for Javascript or Java. This would get you into working on the low-level stuff while still making use of your existing knowledge. Another option is to pretend you are back in school. Find a course with syllabus and homework assignments posted online, buy the book that goes with the course, and do all the assignments. The aggressive approach is to just sit down and write your own boot loader or maybe jump into a capture the flag (CTF) problem with IDA Pro. In case you want to learn from a simple OS, look into xv6 from MIT. Add a few features, possibly as suggested for various courses that use the OS for teaching. You could add a debugging interface. Another idea is to write a disassembler.