Love this book and associated materials. I really wish there was more however.
More advanced cpu design. (pipelining/superscalar)
or perhaps extending the toolset. Adding a simple timer to the hardware simulator would allow some education about better/worse hardware designs and their effects on the end processor speed.
Is there anyone working on and or interested in this?
My personal interests would steer me toward higher levels of abstraction: further development of the OS, the creation of a streamlined VM/CPUEmulator (for just running the programs, quicker than the provided emulators), and perhaps creating your own programming language rather than Jack.