Well, Quartz is okay too, even though it lacks network transparency.

Arcan is looking like a very promising successor to the X protocol and Xorg server. It's not there yet, but it gets closer year after year.

I think Linux (and other unix-like system) would be best off with something like rio on plan9. The API should look like a file-system hierarchy, and API virtualization/access management/input output hooks and mapping could also be file-system-like.