With the availability of all sorts of affordable controllers and touch interfaces it is increasingly common to control a system without the need for visual feedback from the main GUI.
I think it would be very cool if you could run a max patch without any UI. You could use the standard UI for building and testing the patch, but once it's finished you can then do somethings like this from the commandline:
$ open mypatch.maxpatch --args no-gui-mode
I think this is even more relevant with Max5 being such a resource hog when it comes to ui graphics. Just think about all those lovely cpu cycles which can be reserved for audio, video and other important things.
This also opens up possibilities for starting and controlling max in the background without anything appearing on the screen. It's so easy to make Max an OSC server/client to still have full control over whatever. For Jitter there would still be the need to render jit.window, or have some other way of outputting a render context, since otherwise this feature would be useless for video performance / installations.
I really think this could be great. I hope it's not too exotic or difficult to implement.