I make the specification of non-linux because otherwise this would just become a thread full of obscure distros that do the same thing as a million other distros.
Some lesser known OSs:
- AROS - based on Amiga OS, has some derivatives like IcarOS and MorphOS
- Haiku - based on BeOS
- Redox - Unix-like, made in Rust (might technically count as linux?)
- Serenity - Unix-like, very late 90s look and feel
- Kolibri - Tiny OS, the image is ~44MB. It also has a smaller version that fits in a single floppy.
- PhantomOS - When 3 Russians decide to turn everything about a typical OS upside down.
I would travel back in time and make Acorn RISC OS go mainstream. It’s a light OS with a full desktop environment, capable of playing video and rendering anti-aliased fonts, that boots all from ROM and runs on an 8MHz ARM CPU and 512 KB RAM.
I said “go back in time” because I’m not sure how advanced it is compared to modern systems, it’s single-user only and has cooperative multitasking, instead of preemptive.