The interface design needs to be advanced to allow for more commands and actions.  The design should be flexible to allow for more features and not disagree what is already there.
Right now the interface is minimal using a few keyboard and mouse actions explicitly stated and combining them in intuitive ways.  There are not different drawing modes or viewpoints, and there is actually an intentional dearth of features or options.

Some examples of capabilities that are wanted are: creating static links to view-only versions of a drawing.  Creating static links to a view-only static version of a drawing.  Adding the ability to create and delete closed surface polygons.  Undo and Redo.  Inserting text into the drawing.  Using bitmaps to compute font geometrical outlines. Importing and exporting static data.

And some more fancy potential features (could go in seperate products):
Using scripting with the drawings.  Both use drawings to edit scripts and scripts to edit drawings.  Defining volumes and rendering surfaces and other images from volumes.  Using volumes to compute surfaces.

The interface design expansion/new generation should incorporate the most important features above nicely as well as allow for other similar features that could be concieved in the future.