While I completely agree that NetBSD's Rump kernel would be a great way to go (I've played with it in the past and quite like it!) I do wish there was a similar initiative bound to the Linux kernel.

The closest I've seen is Octavian Purdila's Linux-Kernel-Library project: https://lkml.org/lkml/2015/11/3/706

Not sure how far they've come though. Perhaps Kerla could be a prime mover for them ?