I agree with the idea of GDL graphic editor but it is not so easy task.
You might say well Revit can do this... well, if you wanted to create something at the level of parametric complexity as eg AC standard library door or window in Revit you would probably poop your pants.
It can get very complicated sorting all reference planes and dimensions graphically and you can end up (and you will) at the point of an absolute mess where nothing works at all.
Simple and even parametric objects can be done by copying/pasting or saving model from AC. You can still add a few lines of code (I am talking about literary 3 or 4 lines of a simple code) to make it more parametrically flexible.
::rk