Well, there were Momenta and PenPoint --- the latter in particular focused on Notebooks which felt quite different, and Apple's Newton was even more so.

Oberon looks/feels strikingly different (and is _tiny_) and can be easily tried out via quite low-level emulation (and just wants some drivers to be fully native say on a Raspberry Pi)

Maybe a catalog of kernels?

There's a non comprehensive list of hobbyist kernels at https://wiki.osdev.org/Projects

MercuryOS towards the bottom is pretty cool

MercuryOS [1, 2] appears to be simply a "speculative vision" with no proof of concept implementation, a manifesto rather than an actual system.

I read through its goals, and it seems that it is against current ideas and metaphors, but without actually suggesting any alternatives.

Perhaps an OS for the AI era, where the user expresses an intent and the AI figures out its meaning and carries it out?

[1] https://www.mercuryos.com/

[2] https://news.ycombinator.com/item?id=35777804 (May 1, 2023, 161 comments)