We have been waiting to see if we start to see any patent applications that suggest a full frame short registration distance optical design, and here's one patent application that may indicate something of what Canon is planning.
This is a 16-28 2.8 patent application of a full frame lens with a short backfocus of only 11.34mm. Suggesting a full frame mirrorless with a shorter registration distance than the EF mount.
This patent application is specifically for shortening the backfocus of the element design, making it a solution for a future short backfocus mirrorless camera system.
Japan Patent Application 2018-077320