DragonFly BSD, the only remotely functional open source BSD project on this
planet, has plans in place to push certain kernel components like their VFS
layer into userspace for easier debugging, testing and other things that go
with developing file systems easily. If they back it with something like C++
for doing constructor style type conversion on top of overloaded operators
to back VFS data structure translation, could easily import stuff like most
Linux file systems without major restructuring, say, if they had their
translation library written. In this case, userspace kernel systems have
some serious programming advantages over traditional kernels.
They're also pushing an async syscall model to support a non-1:1 threading
system for userspace unlike what Linux has done with futexes. It'll allow