Why would you crop it to a "normal" AR?
And what is a "normal" AR?
3:2 because that's what 35mm film used?
16:9 because that's what wide screen TV uses?
4:3 because that's what a lot of computer monitors are?
If a 3:2 is always cropped in order to fill a 16:9, then what's the point of a 3:2 sensor?
If a 3:2 photo never fills a 4:3 or 16:9 screen, what's the point of a larger screen?
The point here being that aside from historical bias towards 3:2 there is no reason for digital photographs to be that way.
Aside from the legacy of 35mm film, there is the proximity to the golden ratio that favours the
3:2 aspect ratio. (Actually 16:10 would be closer.)
I get the point though of asking "what is a standard aspect ratio?"
Take a look at a wedding album - you will see all sorts of aspect ratios that can readily be printed.
What will be a technical consideration in working with a square sensor is that the sensor will be
higher (even if only 30.6mm) - quite a bit higher than 24mm. That means that the reflect mirror
and pentaprism need to be totally redesigned (assuming a SLR design).
It has even bigger implications for the shutter. Building a focal plane shutter than has to travel
30.6mm will probably be quite a technical challenge - especially when you want to maintain
a reasonable flash sync speed. I doubt that 1Ds buyers would accept anything less than 1/250sec.
My uneducated guess is that with the same curtain travel speeds, you would not be able to
reach 1/200s on a 30.6mm high sensor. (I stress - uneducated guess
.) By increasing
the travel speed of the curtains, you increase the mechanical stresses, which means durability
becomes an issue. Another way around that is to go back to an electronic shutter, which however
is a compromise from an image quality perspective.
Canon would need to do a great deal of engineering to change the sensor aspect ratio. That
said, there has been a long delay since the launch of the 1Ds3.