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.

In the event that I do need to use something that is Wayland-only, I'd like to have the ability to run it without breaking everything else.

I don't know of any significant applications that are Wayland only. If that ever happened, someone could just make a really simple Wayland server that does the reverse of XWayland.