Canon Rumors has something about a rumoured Canon full frame mirrorless:
We have now confirmed from a couple of good sources that a full frame mirrorless camera is well into its development cycle, as the camera is being used by select Canon pro photographers.
One of the biggest unanswered questions about Canonâ€™s upcoming camera is whether it will feature a native EF lens mount. Allowing Canon photographers to seamlessly use their entire existing arsenal of Canon lenses would be a huge boon to the cameraâ€™s ability to disrupt the market.
I wrote previously whether a SLR mount on mirrorless would make sense?
The answer is a NO.
What I see, if that camera actually exists, is an EF-M mount for full frame, possibly compatible with the actual EF-M downward (full frame on APS-C) but not the other way due to coverage, similarly to the EF-S mount cameras, that can use EF mount lenses. Like for the EOS-M, I also see an adapter available for EF lenses.
Time will tell if that gets to be beyond a rumour. At this time I have no speculation about its specs.