Or maybe Minix3, XNU, L4, or Mach?

http://www.minix3.org/

https://en.wikipedia.org/wiki/XNU

https://en.wikipedia.org/wiki/Mach_(kernel)

https://en.wikipedia.org/wiki/L4_microkernel_family

Might as well write it from scratch without POSIX and all the lessons we've learned since then.

I think rust opens the door to designing kernels that are small and tight with most of the OS stuff that was traditionally in kernel space moved into user space.

Opens which door? Minix3 is microkernel based and userland compatible with NetBSD. XNU is originally based on the Mach microkernel and has part of FreeBSD bolted on. Both are open source, as are Mach proper and L4.

Perhaps someone could start with Minix3 or Darwin rather than Linux or BSD or from scratch. Replace one component at a time in Rust, or D, or Ada...

Easily machine verified kernels that have a high degree of having no security exploits because of buffer overruns or null pointer shenanigans etc...