I'd still like to have QNX-type messaging. The UNIX/Linux world started out with no interprocess communication, and ended up with a large number of incompatible ways to add it. The Windows world started out with interprocess communication with everybody in the same address space, and gradually tightened up. QNX started with messaging as the main OS primitive, and everything on QNX uses it.

The key to efficient IPC is that the scheduler and the interprocess communications system have to be tightly coupled. Otherwise you have requests going to the end of the line for CPU time on each call, too many trips through the scheduler, and work switching from one CPU to another and causing heavy cache misses. QNX got this right.

(Then they were bought by a car audio company, Harmon, and it was all downhill from there.)

QNX messaging isn't a "bus" system, and it has terrible discovery. Once communications are set up, it's great, but finding an endpoint to call is not well supported. The designers of QNX were thinking in terms of dedicated high-reliability real-time systems. It needs some kind of endpoint directory service. That doesn't need to be in the kernel, of course.

QNX is a microkernel, with about 60KB (not MB) of code in the kernel, and it offers a full POSIX interface. (There used to be a whole desktop GUI for it, Photon, good enough to run early versions of Firefox, but Blackberry blew off the real-time market and dropped that.) File systems, networking, and drivers are all in user processes, and optional. L4 is more minimal, probably too minimal - people usually run Linux on top of it, which doesn't result in a simpler system.