The current GUI designer in Blockland, in which I have dabbled a little, is sorta already like this. You just select your GUI, make a new object, whether it be a button or input box, then add the new object to the GUI. I wish someone could make a more comprehensive GUI making tutorial, because I have all the basics worked out, I just need to know the advanced stuff for what I'm doing.
That would be very useful, and user-friendly to encourage more people to design GUIs.
Probably would not be practical in-game - would be better as a separate program.
Definitely.