Actually, the cow will vary in size. Packages with classes those don't share superclass will be represented with big big big cow. That cow will be hiding most of other 3d objects with its huge back. And some cows will be small like an ant. Besides, cow will genreate lots of polygons.
I have no idea presenting both hierarchy and package membership in one 3d space. All I can think of is filtering in classes with user selected package.