I don't understand the categories. How could it be possible to get more mainstream than the current Model S styling?
I expect the GenIII to look like a scaled down Model S complete with a cramped interior. I would prefer it to be something other than a three-box design so that interior room isn't sacrificed to the gods of boring.
I am with you about Gen III getting away from the 3 box look. The main issues I see are crumple zones, and aerodynamics. The front compartment allows for both of those, and the trailing trunk/hatch is pretty useful/areo at the same time.
I am hoping for something with a short nose, and a hybrid between a wagon, and hatch back (that tapers down). The only issue is rear visibility with the roof obscuring vision.