Jason Ekstrand laid out his proposal on Friday for a system compositor protocol and is seeking comments on his proposed protocol and concepts surrounding this system compositor. He already has his own Weston fork that does implement these ideas.
The basic premise is providing an interface for compositors where all they end up doing is displaying other compositors or full-screen interfaces. The primary use-cases would be serving as an abstraction layer for other back-end/server implementations, simple full-screen client support, and providing a DRM/KMS back-end for other compositors.
Those wishing to learn more about this Wayland System Compositor Protocol can read the mailing list post and check out Jaston's Weston code.