I don't want an X server that supports Wayland. I want a leaner, simpler X server without Wayland.

EDIT: To clarify, the reason I mentioned Wayland compositors is that it'd be an opportunity to pick a low-level rendering backend that has been written from scratch without the baggage of Xorg.

The "good parts" of X that modern apps actually use are comparatively simple compared to the low level bits - the protocol is trivial-ish, and you can get 90% there by implementing a small-ish subset of the protocol.