I really wish all these new Mirrorless systems would have used 35x35mm sensors. You’d think we should be able to move past the idea of physically flipping your camera around all the time.
Really? At the cost of making the camera bigger (taller for both the extra size of the sensor and of the viewfinder), heavier and more expensive; and needing a whole new suite of bigger, heavier and more expensive lenses?
Note that current lenses are designed to cover a 43mm diameter image circle while your format will need a 50mm diameter image circle. That's 35% more area to cover.
Many current MF cameras have sensors of 44 x 33mm with image circle of 55m. So your suggestion puts size (and everything that follows from that) more than half way between FF and MF.