You'd have to re-implement the Mac graphical API (cocoa, I believe it is called). It should be possible to run Xorg on a GNU/Darwin system, but don't expect any Mac applications to run at all. This does not include the graphical stack (the equivalent of Xorg or Wayland). What Apple is giving you is basically the equivalent of a Kernel + some core system utilities and drivers. However, you need to set you expectations right. There are already some projects based on Darwin, and you can build your own GNU/Darwin (or whatever userland you prefer/Darwin) distro if you have the patience.
