I'd be down for an Erlang-like OS ABI (or, to put that another way, a Windows-GUI OS ABI, but for even non-GUI processes): just message-passing IPC all the way down. OS signals, disk IO, network packets, [capabilities on] allocated memory, [ACL'ed] file handles, etc: all just (possibly zero-copy) messages sitting in your process's inbox.

Of course, it's pretty hard to deal with a setup like that from plain C + libc + libpthread code, so OSes shy away from it. But if your OS also has OS-global sandboxed task-thread pools (like macOS's libdispatch)—and people are willing to use languages slightly higher-level than C where tasks built on those thread-pools are exposed as primitives—then it's not out of the question to write rather low-level code (i.e. code without any intermediating virtual-machine abstraction) that interacts with such a system.