A question to the more scientifically competent than I:
So, DSLRs have bigger lenses than rangefinders or other mirrorless cameras because there is a bigger distance between the end of the lens' flange and the sensor. Would it be possible for Canon or Nikon to make a mirrorless medium format camera that could use their existing stock of 35mm equivalent DSLR lenses?
Lens size really has nothing to do with the size of the sensor behind it, it's all about light gathering. Flange distance can make a difference in that if you need a focal length shorter than the flange distance you need a retrofocusing lens design, which adds a lot of elements, otherwise it makes no difference.
The EF mount has a 54mm internal diameter, the Nikon F mount has a 44mm external diameter (the whole thing fits inside the EF mount. I can't imagine sticking a supertelephoto lens on something that small).
Nikon is basically SOL, but Canon, in theory, could use their existing mount with a sensor the same size as the Leica S, and just 1mm short of the Pentax 645.
Those lenses would still work with 35mm bodies, and as long as you don't mind cropping I see no reason an EF lens wouldn't work on a 35mm+ mirrorless EF mount (I still hate calling these "medium format", they never would have been called that in the film days).
And from a 35mm point of view, that would give us well more than the space needed for a "square" sensor that never needs to be held in "portrait" orientation.