I think it's possible we might see two versions though I voted for EF-M mount as I think we'd see that first. My thinking is they won't go all in and replace their DSLR lineup with EF mirrorless until they have a viable working solution with A solid EVF.
Replace their DSLR lineup with mirrorless?!
That is a much
further out proposition.
IMHO, there is zero chance Canon's first FF mirrorless is a straight swap for a current FF SLR line. Forget mounts or cannibalization, it will be an inferior product to the corresponding SLR it has similar features to, so to a devoted multi-generational 5D2, 5D3, 5D4 owner, a mirrorless 5D4 will be a takeway rather than an add (with current batteries, current EVF tech, current responsiveness, etc.).
So I see Canon's first FF mirrorless (interchangeable) option being an altogether new brand sold alongside SLRs.
Over time, of course, that could change. But the idea that a 5DS2 or 5D5 will be mirrorless only is nuts. A 5DS4
, maybe, but certainly not the next model of each.