A performant fork - the single biggest problem with Cygwin - sounds awesome. As does the ability to run Linux binaries directly, opening up the direct use of binary package repos.
I wonder if I can use it via mintty instead of conhost.exe; at the very least, I could ssh into it from Cygwin.
As always, the devil is in the details; the rough edges where support peters out, or syscall inconsistencies creep in.