With shape and size adjustments, we can make some pretty interesting fixtures. I think it would really open up some design possibilities to have some more 3D adjustments for depth and tilt to form the body around them. A couple examples:
Create a front end the slopes down near the forward end. Place the round, chrome-trimmed, headlights and adjust them to a bit of a vertical oval shape. Now tilt them forward/up to create a new raised section of the hood flowing back from the headlights. (a LA Aston DB5 among many other beautiful cars)
Create a flat-faced design and place a grill across the width of the front end. Then increase the depth to create a deep indentation for headlights. (ie 69 Challenger)
I realize this would take a lot of modelling magic but it would add a huge amount of flexibility without the need for a longer list of base body shells. It would also allow for some great period-correct design elements like the big tail-fins in the '50s.