Obviously the best solution would be to have a circular sensor with the circumference touching the image circle. No more turning the camera 90 degrees for portrait, and you could set the dimension percentages for anything you like!
Of course, many lens hoods would not work, and some "L" lenses apperar to have a baffle that may limit the vertical part of the circle.
Then, there is the cost to make the sensor. It would have much more area than FF and might cost XXX the price.