What about a pivoting or sliding roof where it is a fastback when seating only 2 people (in the front seats) then you may rise the rear roof section when you need to seat people in the rear seat?
Kind of a CRX when seating 2
Turning to a Civic when seating 4 or 5
Also kind of Jeep Wrangler fastback tops
Turning into normal square tops
Did you get the idea?
4 door sedans would be tricky to design while still looking good.
As to avoid ugly designs like the Porsche Panamera
Its cheap copy the Honda Accord Crosstour
And even the BMW X6
Probably despite the aerodynamically inclined roof when seating 2, saloons or coupes will still need a separate trunk/boot section?
Suggestions?