I would love to see some examples outside of the WIMP-based UI

This list should include SerenityOS IMHO.

It might not be super unique but is a truly from-scratch "common" operating system built in public, which for me at least puts it at the position of a reference of an OS of whose code one person can fully understand if they'd want to understand the codebase of a whole complete-looking OS.

I had the privilege to work as a junior operator in the 80’s, and got exposed to some strange systems .. Tandem and Wang and so on .. and I always wondered if those weird Wang Imaging System things were out there, in an emulator somewhere, to play with, as it seemed like a very functional system for archive digitalization.

As a retro-computing enthusiast/zealot, for me personally it is often quite rewarding to revisit the ‘high concept execution environments’ of different computing era. I have a nice, moderately sized retro computing collection, 40 machines or so, and I recently got my SGI systems re-installed and set up for playing. Revisiting Irix after decades away from it is a real blast.

Too much time on your hands folks. Get out of your cave, enjoy time with family, friends.... life is so short to lose time on designing sth just to make a post on HN .... or God forbid, X.com...