Users are well versed in editing polyline and polygon based building elements using the tools in various pet palettes. They know how to add and remove points, make lines into arcs, offset single edges or the whole shape. However if a custom object uses polylines or polygons, they can't use these tools and must use whatever the GDL developer has managed to hack together in order to imitate them (for example the Dynamic Polyline objects).
I propose that the standard pet palette tools should somehow be made available to be used to edit custom GDL objects. Maybe there could be a new parameter type "polyline" or "polygon", whose presence in the object's parameter list would automatically trigger the pet palette? Maybe the polyline/polygon parameters could be just dictionaries (like the ones used by the Polyops add-on), and in GDL you could indicate that a parameter is an editable polyline/polygon? Maybe the declaration could include finer settings about what kind of editing can be done: can there be arc segments, holes, multiple polygons. Maybe even allow 3D editing or define the plane of 2D edits.
Polylines and polygons are so ubiquitous in all kinds of design that this would be a natural next step from the regular length/angle editing with HOTSPOTs.
Open