The kernel and the C library as a single project

There are libraries that do this today. A couple of examples:

libpcap abstracts the OS-specific calls to setup and capture from a raw socket. It includes several variations on a theme for Linux specifically, ranging from select/read semantics to mapping a shared-memory ring buffer.

libevent abstracts OS-specific replacement syscalls for select/poll, using whatever is fastest for a particular OS.

libaio is similar, but for asynchronous IO

These libraries all present a stable ABI for apps to use, similar to libc. Having a library like this seems like the perfect incubator for new syscalls as well.