As Canon already has a mirrorless mount that will work with FF it's my guess they might just use that.The OP raised a question about what mount Canon might use for its new FF mirrorless (if in fact they do anyhting. I'm suggesting they might use the EF-M mount which they introduced for APS-C mirrorless. If they do that then its my assertion that Nikon will then be the only camera maker with a mount specifically designed for FF mirrorless. I'm not really saying anything else.
So what Sony and Canon have done, both of them, is design a mount that suits APS-C, and then shoehorn a FF sensor in later which just fits. I'm sure they had plans along those lines in the early design stages, but nonetheless that's the order they did it. Nikon have come at the new mount design in the reverse direction, favouring a design that specifically suited FF.
It's also possible that Canon could use the DSLR EF mount dimensions (54mm diameter) and shorten the flange distance to 16mm like Nikon. Unlikely but. Let's wait and see
I wonder if they might surprise us all by sticking with the regular EF DSLR mount along with the 44mm flange distance. That way their whole line of pro DSLR lenses will work. They can produce new lenses that extend into the body if they wanted.
With full frame and 1.4 lenses the body needs to be large enough and needs enough space between the grip and the lens for fingers to be comfortable.
I think most full frame users have gotten used to the idea that FF mirrorless is not really allowing for smaller lenses so maybe a 6D sized mirrorless body that uses the full range of FF Canon DSLR lenses without an adapter may be a good idea.
Canon will still outsell everyone else and Canon FF DSLR users will be happy they can use their old lenses still.
