Virtually Grey

Several of the preceding suggestion, but above all ...

A circular sensor (7072 px radius) would allow a square image of 5000 x 5000 pixels. The aspect ratio would be selectable to any that fits the circular sensor. So, no rotating the camera for portrait format shots - instead select aspect ratio via (yet) another controller.

A circular sensor allows the maximum pixel dimensions for any given aspect ratio. Presently I lose 1/3 of the pixels from my 7D sensor if and when I crop to a square.