We Have Mir & Wayland, But There Still Could Be X12

[JS987]

Would something like OpenVG corresponds to what you are thinking about?
If it's below the toolkits, and implemented by all drivers, it will end up being some "Khronos group" API style, especially if not "NIH".

It is question for toolkit and graphics driver developers whether OpenVG is good option. I guess it is better than nothing.
It seems only Gallium drivers have some support. Problem would be to convince all toolkit and graphics driver developers to support it.

[mrugiero]

Any toolkit can use xrender on X, but if 2D driver would be e.g. Cairo backend on Wayland, non-Cairo applications / toolkits can't use that driver. I can't force toolkit developers to use Cairo.

OpenGL applications can hog CPU and GPU at same time.

You can make an agnostic 2D driver, exposing an API for toolkits to use if they want, kind of like xrender.

Can you show an example of OpenGL applications that hog CPU and GPU and aren't games (since those are beyond the scope of window management, and are almost always pretty simple in the window side of things)?
AFAIK, most X compositors use OpenGL. There are some of them using the X way (XFCE, a backend for Kwin, and compmgr, I don't know of any other).