I think working with a ready-made Unix isn't as friendly to the idea of 'hobbying' an OS. For studying how Unix works, it's great. But if I wanted to make experimenting with my own bespoke system worth it, I would try to not have preconceived notions about how it should work, or copy an entire design. There are dime-a-dozen Unix-workalike hobby OSes out there, and in the shoes of an experimentalist there's a lot to weigh you down in the Unix ecosystem, and a lot of things worth trying that break from the way Unix does things.

If you really did want to work with an existing OS for actual research/experimentation purposes, and you're a fan of C, I would go with Plan 9/9front. There's a reason Bell Labs ditched Unix--because it wasn't a worthy research platform anymore.

Fair enough. I know some open source hobbyist OSes which are non-DOS/Unix variants... sadly they are X86 only, like Visopsys or Kolibri.

For something non-X86, try digging at: http://wiki.osdev.org/Projects Perhaps you can find something suits your preference.

>The old xv6 is x86-based and it's not officially maintained anymore.

Which is sensible. There's way too much legacy crap and ugliness that needs to be dealt with in that ISA to distract from the purpose of the project, which is to teach OS development.