I've proposed many times before, a "universal" 42X42 sensor. The size increase is not much, but enough big for when cropping to rectangular (if desired), you end up with an effective sensor size bigger than a full-frame dslr.
This modest size increase would probably yield also a modest increase in cost. But the real beauty of it would be that being a 42mm square sensor, it would fit tightly in a 645 camera.
So we can have a dback for 6X6 cameras with a square sensor not much smaller than full frame, something around a 1.3X crop factor. Very importantly, it would allow a 645 camera to be handle as a square camera to use it as a full-time waist level finder style and without ever turning the camera for horizontals to verticals. That's why I call it "Universal". A "convenient compromise" between an ideal sensor for 6X6 and an ideal sensor for 645.
"Convenient Compromise", "Ideal" where convenient and ideal mean truly affordable.
If the idea could be accepted by consumers, this sensor could be in digital backs for the thousands of "square" and 645 medium format cameras already sold for decades.
37mm square sensors have been around for ages and work very well on a 645 body. You have to get used to using shorter focal lengths than you would with 6x6, but otherwise the experience of shooting with them isn't really much different to film once you get used to the vagaries of the back itself. I have been using a square-format, 16 megapixel monochrome back made by Megavision on my Bronica system for a year now -- I've had some great results and it continues to be a pleasure to use.