@ runxel
If someone has done it, it's a good sign.
I came up with this idea after seeing some limitations of the LPM (Library Part Maker) addon for creating "Stretchable/Parametric" doors/windows. This new commands would allow the creation of custom parametric objects with the "Save selection as Object/Door/Window" command. The user could define a morph plan and change its ID to something like "StrchPlan_MyNewParameter" and the command would convert the morph geometry to a GDL StrchPlan and add the parameter to the object (Similar to the WallNiche solution in custom windows).
@Lingwisyer
A new GDL dialogue would also be great. Then, users would have a more "friendly" interface to refine it.
But always with a "Hard" command for complex GDL scripting.