I was just thinking today that a transparent cube with just the outlines solid would be so useful. Sort of a wireframe view. OR the ability to apply a texture to one or more sides for the cube--or not.
As for other shapes we kinda need ... a quarter circle (hollow not solid) is top of the list for me. A hollow half-sphere would also be very useful. Thinner walls to the hollow cylinders (all variations) would be sweet. Very hard to create a realistic vessel as is.
Holy crap a wireframe view option for building objects (especially small ones) would be amazeballs.