

Should be possible, as it’s a normal VM you can already install flatpak apps in said VM as normal, you’d just need a Windows side bit to invoke the install within WSL when you opened the flatpak bundle, and then something to add a start menu shortcut from the app inside the VM (Which I actually assume already exists, I never actually ran WSL2 when I was on Windows)
Plan 9 is inspired by UNIX (Helps that it had the same devs), but it’s not a direct continuation.
UnixWare is I think the only direct continuation of the original AT&T UNIX. The various BSDs are close enough but were re-written entirely in the late 80s/early 90s so there’s nothing original remaining.