One thing to note here, is that for any infant system to have it's best shot at thriving, it should be built from scratch without any compromises or hindrances of compatibility to an older system. One thing that appears in Leica's marketing mumbo jumbo is how they didn't compromise the M system because the camera was completely blah blah blah, here read it yourself..
"The 18-megapixel CCD image sensor, specifically designed and developed for this camera, enables the capture of the full 35-mm film format without any compromises. All M lenses mounted on the M9 or M9-P offer the same angle of view they had when using film material. This means that the high resolution and unique imaging quality of the now-digital M9 and M9-P are capable of fully exploiting the enormous potentials of M lenses. Or, in other words: in the case of the M9 and M9-P, it wasn't a matter of modifying the lenses to match the image sensor, but rather the other way around. This results in images particularly rich in details. Natural. Authentic. Without compromise."
So, in theory, to be even less without compromise, a mirrorless format might do well to be completely designed around new lenses and bodies without any consideration to compatibility to the EF or EFS mounts. This would probably be wise weather Canon goes for FF or for a crop variation because EF lenses in particular dwarf those of FF Leica or crop micro 4/3rds and crop Nikon 1 series mirrorless efforts. There is also to date (but this won't hold forever with new in camera softwares) no great reasons other than ultra small sizes and weights to go fully mirrorless. So compatibility with large and or heavy lenses should be IMO the absolute last thing on Canon's list if they choose to enter this market. Could you imagine hooking up a 24-70L to a small micro 4/3rds system or a Nikon 1 body, or even a Lieca, it would be a disaster.