For those asking for full frame it's not happening any time soon and if it does than I'd expect a minimum of 3499 and a non EF mount due to the flange distance is not suitable for anything remotely compact. I'm sure an EF adapter would be possible but pretty much the best you are looking at is Leica M9 @ $4000 with autofocus and a new lens lineup that is not EF compatible and at canon L prices.
I'd much rather see something comparative with the fuji xe-1 which has pretty much the best aps-c sensor
Good AF, good lens lineup size, great control scheme and awesome styling
Any camera with a normal EF mount would still have the same flange distance as current EOS DSLRs. With a mirrorless design, all you would lose is the bulge of the pentaprism (I am assuming a pentaprism, since all full frame EOS bodies use a pentaprism).
To get any size benefits on full frame would require a new lens mount - or at least different optics, which allow for the rear element to be placed closer to the sensor. In addition, the sensor would need to cope with incident light coming from a more oblique angle - something that digital sensors do not do well. Leica has had to do some clever things with its micro-lens arrangement and IT and low-pass filters in order to get the lens closer to the sensor - something that was not a problem with film.
Even with the 85mm f/1.2 lens, Canon is doing some fudging of the sensor sensitivity because of the oblique angle at which the light strikes the sensor - the sheer size of the rear element is something to behold.
Given current technology, a full frame mirrorless camera would not give any size advantage over a DSLR. Given that OVFs are superior to EVFs, that begs the question, of why even to bother with a full frame mirrorless camera.
In my mind, APS-C mirrorless makes sense, full frame doesn't.