I ran BeOS back in the day (even have the developer book!) and I've been trying Haiku on and off over the years. It's been interesting. The browser isn't quite all there yet but might be considered serviceable, and you can sort of get a working dev environment going on it (not many modern runtimes, though, nor a way to run Linux binaries that would let me do Clojure). It's certainly worth keeping an eye on, although…
The pi is such a great device for this. If I were working on any niche operating system (or building one from scratch), I'd target qemu first, and the pi second. It may not be the nicest hardware out there, but it is a single platform that loads of people have, that allows trivial disk swapping (upside of no onboard flash -> everything on swappable SD card), and is dirt cheap.