Components with a standard footprint prompt the user for the dimensions of the physical package. This then appears as a rectangle in the layout view, and is exported as 3D when generating the stl. However this is not possible to edit for components with non-standard footprints, which makes in my experience the stl export useless, where it could be of huge help.
So it could be cool to be able to define for any component the physical package. This would require defining the width and height in the layout, as well as the height above the PCB.