Back in the 1980s, operating systems and compilers were often seen as the most complex pieces of software. Nowadays, web browsers include (and reimplement) large parts of an OS and compiler and are probably even more complex than a current OS such as Linux or a current compiler such as clang/LLVM. Only rich (or well funded) companies such as Google, Apple and the Mozilla foundation can afford to build a browser today that can be used to access current web pages.
So a central question is if we can turn back times and make software more simple again. Maybe this ship has already sailed - but it can't hurt to try. From experience with my students, it is extremely satisfying for them to build a complete system from scratch instead of mostly copying and pasting library calls or StackOverflow code snippets. Thus, I try to enable my students to experience this sense of achievement. They will probably never get the chance to do something similar in their later career in industry.
[1] One Man Unix for 68k - http://www.pix.net/mirrored/discordia.org.uk/~steve/omu.html
[2] Uzix for Z80 (link to the MSX port) - http://uzix.sourceforge.net
[3] Coherent Unix - https://en.wikipedia.org/wiki/Coherent_(operating_system)
[4] Minix - https://www.minix3.org
[5] Alan Cox' Fuzix - https://github.com/EtchedPixels/FUZIX
[6] Ardi Executor - https://en.wikipedia.org/wiki/Executor_(software)
It's roughly the same idea, but actively maintained and runs on a wider range of hardware.
[1] https://github.com/EtchedPixels/FUZIX
Edit: Surprisingly, many still on eBay. Just bought one for $19.65, in the original box, including shipping and tax.
Grant Searle's breadboard CP/M machine: http://searle.x10host.com/cpm/index.html
Spencer Owen's RC2014 modular bus-based Z80 system: https://rc2014.co.uk/ Highly active RC2014 discussion group: https://groups.google.com/forum/#!forum/rc2014-z80
Phillip Stevens' beautiful YAZ180 Z180-based SBC: https://github.com/feilipu/yaz180
z88dk, a Z80 C cross compiler with libraries and all sorts of useful things, which supports many machines: https://www.z88dk.org/forum/
Jon Langseth's LiNC80: http://linc.no/products/linc80-sbc1/
Steve Cousins' many different machines, some RC2014-compatible, some z50bus (LiNC80) compatible, some Z180 stuff, SBCs, a powerful "small computer monitor", etc: https://smallcomputercentral.wordpress.com/projects/
Alan Cox's Fuzix, a unix for 8-bit machines (including many of the aforementioned Z80 machines): https://github.com/EtchedPixels/FUZIX
Most of the folks mentioned can be found on the retro-comp discussion group, which was formed from RC2014 group members to discuss other projects: https://groups.google.com/forum/#!forum/retro-comp
https://www.autometer.de/unix4fun/z80pack/screenshots/2014.h...
Comes with prepackaged CP/M images and can run Alan Cox's amazing FUZIX system:
https://github.com/EtchedPixels/FUZIX
https://hackaday.com/2017/04/16/z80-fuzix-is-like-old-fashio...
This is a nice hack, but not as cool as those projects.
http://retrobsd.org/wiki/doku.php (Related projects).
https://github.com/EtchedPixels/FUZIX
There are other 8Bit unix-likes as well.
Also Fuzix, which can run a Unix-like on a variety of tiny, ancient processors like the Z80 and 6809; I've ported it to the MSP430, a modern micontroller, and I also have a half-finished port to the Raspberry Pi that I must get round to fixing one day (hilariously, the Pi has more RAM than Fuzix can handle)...
https://github.com/EtchedPixels/FUZIX
Both of those run natively, of course.
I am currently reading Uzix[0] and the disassembly of Symbos[1] to do some OS dev on Z80 machines which, in the end, will hopefully run on my own homebrew hardware.
[0] https://github.com/marioaugustorama/uzix-kernel [1] http://symbos.de/
Edit: forgot; https://github.com/EtchedPixels/FUZIX
MSX was a 8-bit computer launched in 1985 that had a great user base (selling 300k units on launch). Its open architecture which originated in Microsoft Japan and ASCII Corp. It had great success also in Europe, Japan, South Korea and others, just like Master System.
This gave companies like Konami, SEGA, Namco, Activision, Taito, ASCII and others a great incentive to develop its famous games for this platform which became predominantly gaming oriented, most units coming already with bundled cartridges and game controllers, particularly MSX Gradiente 1.0.
To port MSX games to Master System was very trivial, given the similarities between both platforms, as follows: Zilog Z80 processor, at 4Mhz, 64kb RAM, 8kb ROM, the only big difference being the video chip.
There are plenty of resources on the Internet and dozens of emulators available and being maintaned. One of the most interesting projects is the recent Alan Cox's Fuzix OS which goal is to "provide a complete System V experience, without the bloat" (https://github.com/EtchedPixels/FUZIX).
Related (you may know it already): https://github.com/EtchedPixels/FUZIX
Implementing a VM is an interesting way to workaround some Harvard architecture's limitations as long as speed is not a factor.