It's not clear what that would solve, XWayland mostly fulfills that role already. It can't run window managers, but you wouldn't really get that everywhere from

It's not clear what that would solve, XWayland mostly fulfills that role already. It can't run window managers, but you wouldn't really get that everywhere from adding another Wayland extension either -- doing that would require a lot of additional code and the GNOME and KDE implementations wouldn't want that anyway because they have their own built in window managers. So maybe you could get a hacked up version of Weston that can run X window managers, but why bother dealing with maintaining that instead of just maintaining X? It's unlikely the X server will stop working as long as you have a GPU that supports GL or Vulkan, the glamor/modesetting driver should continue to work there.