You may find Plan9 interesting. Redox is also something to watch.
In addition to these, the BSD's can be fun, although probably not what you're looking for here.
In addition to Plan9 and the BSDs, which are Unixy and relatively familiar if you're coming from Linux/macOS, you could also checkout Haiku [0], which is quite different and descends from BeOS.
If you have an old TI calculator and want to try something other than a desktop OS, you could try out KnightOS[1].
As Shared404 mentioned, a Unix-inspired but quite new OS is Redox[2], which is written in Rust. Haven't used it at all, but it sounds like an interesting project.