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 Canon’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 camera’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.