I'm probably in the minority by wanting a large mirrorless camera. Canon has certainly said that the majority of buyers want smaller cameras, so there is every possibility that will prevail.
Canon did patent a new lens mount that may have been targeted at a mirrorless.
So, while I'd like a Mirrorless that used EF lenses native, its unlikely to actually happen. Canon will come out with a new lens series, and a adaptor for EF lenses, then trickle out some of the new ones each year.
IMHO, if Canon going to replace some of their DSLR with mirrorless, they can just use EF mount:
1) All current lens will be compatible with it... don't need to spend $$ to create another new mount and new lens.
2) Handling and ergonomics - not everyone want a small camera due to handling. Also, with a small camera body, you can only put so much buttons and dials on it and ergonomics will suffer... a lots of people want fast operation... most don't want to go into the menu every time they want to change something and current Canon DSLR design had very good ergonomics.
3) If you want good IQ with high resolution image, large lens with large diameter lens element is needed and large lens don't go well with small camera body.
4) Mirrorless need more power than DSLR, if you decrease the camera size, battery life will suffer and also to drive the lens for faster AF, you need more power.
Canon can just maintain 2 mirrorless lines:
1) DSLR size mirrorless - for people who don't mind or want a bigger camera body for better handling and ergonomics.
2) EOS M style small mirrorless - for those who want small camera.
Anyway, Canon can just use one of their DSLR design... remove the mirror box and replace the prism with an EVF and they got a DSLR size mirrorless camera...
By the way, I hope Canon can install a some sort of sensor curtain to cover the sensor when not in use (power off or during power saving mode) or during changing lens. An exposing sensor is not a good thing especially when changing lens in the field. Also, imaging a super telephoto lens on a mirrorless during transportation and accidentally point at the Sun... you'll end up a burning sensor.
Have a nice day.