> The fact that that requires implementing Linux kernel functionality is simply an artifact of the lack of a more modular design (or really any up front design) in Linux.

Curiously the recommended syscall mechanism on x86 is by calling __kernel_vsyscall in the vDSO. If everybody did that then you could just make your own loader with your own custom vDSO that could implement the syscalls in userspace. However sometimes (especially statically built) programs still makes syscall directly with int 80h, which are slow to trap in userspace or may not even be possible depending on the os.

Now it would have been fantastic if calling through the vDSO had simply been the only documented way of doing a syscall on x86_64, but the kernel developers at that time decided not to do that, so now on x86_64 the syscall instruction is always used directly, and we can't even trap that on any x86_64 OS because most of the time all that happens is either some unrelated syscall is executed, or an error is returned from the kernel back to the calling process without any traps.

I don't know if there's a single document somewhere with an explicit roadmap. Their GitHub page (https://github.com/Microsoft/BashOnWindows) includes links to multiple sources that could collectively be considered a sort of roadmap, including the Issue tracker, UserVoice page, team blog, and discussion forums. I've also asked the devs questions on Twitter a couple times (like @richturn_ms) and usually gotten fast responses.