Canon has one major advantage over Nikon, if going mirrorless with a short flange mount, and that is the fully electronic mount. This means that Canon can make an ekectronic adapter without mechanical linkages, in the same way that Olympus made the MMF-3 adapter for 43 DSLR lenses to the micro43 mount. This is a much better and even more seamless solution than any of the adapters for the Sony E mount, e.g. Metabones, Sigma MC-11, LA-EA3/4. Nikon on the other hand has a plethora of lens types with and without aperture rings, with and without AF motors, and only a few fully electronic E type lenses. Nikon will face a hard time supporting their lenses on mirrorless. Canon doesn\'t have to reverse engineer anything, and they can make a very good seamless adapter solution when going mirrorless.
