The square format rumor still confuses me every time I hear it... That would not be a good choice. While I do love cropping many of my images to square after the fact, it's an impractical format out there for so many professional users.
square was the professional standard for medium format before digital for hasselblad users. it prevents you having to turn your camera around for portraits. you do have to be mindful of cropping in post while you're actually doing the shoot though. It also works better with tripods.
Square was the format for Hasselblad and no one else. As it was, that was a little impractical for a lot of things since you'd almost always have to leave extra space for cropping. The, "flipping to portrait," issue only made sense when we were using waistlevel viewfinder, an issue that Mamiya had no problem fixing with their rotatable film back on the RZ series. Now days, we simply use an L bracket so that the centre of gravity is still directly on the tripod.
I think if Canon were to not go with the 645 format, they'd be screwing themselves.