Beyond Canon's stellar brand loyalty and brand power, there are a few things that a full frame Canon mirrorless absolutely needs to be successful.
Fstoppers posted an article today outlining what they think it needs, and it pretty much mirrors (doh) what I've been saying for the last year or so.
A native EF mount
The Canon EF lens lineup is the best in the business. No other camera manufacturer has the breadth and depth of lenses, nor the manufacturing power to churn out amazing glass time and time again. This is Canon's biggest advantage over other manufacturers and will ensure they take the #1 spot in mirrorless camera sales without much marketing effort.
This is where I think Nikon is going to make a massive mistake if they really do require you to need an adapter to fit F mount glass.
I hate adapters, and I even hate teleconverters. Any extra thingy I have to worry about to use gear is a non starter for me. If I think I have to add a teleconverter to a 500mm lens, then I'll buy a 600mm lens. Thanks to Canon for the EF 200-400mm f/4L IS 1.4x.
Lens adapters are to solve a unique problem, and I don't mind them if I really have a need to use an FD lens or a Nikon lens on my Canon camera. But if I'm using a modern Canon lens, it better fit without an adapter on my modern Canon camera.
Adapters are lazy.
Size and weight?
Is this really why we want mirrorless cameras? I'd argue going too small is actually bad for usability when attaching larger, heavy lenses. Attach a Canon EF 85mm f/1.2L II to a Sony A7R with an adapter. It's all sorts of awkward to shoot with and you get odd fatigue in your hands, wrist and forearms. If you're adding a battery grip to your mirroless camera, then you're already admitting you don't care about size and to some degree, weight.
If you remove the mirror and shutter assembly, you're going to naturally reduce weight.
Here's something for you, a Canon EOS Rebel SL2 weighs less than a Sony A7R with a mirror and shutter assembly. Yes, the build quality is different, but I'm sure Canon can come up with something to make a camera with their great durability and amazing weather sealing.
The EVF has to be the best in the business. Period. No skimping on parts and manufacturing here Canon.
It's rare I give my opinion on gear, but I'm actually quite opinionated on what Canon needs to do with a full frame professional mirrorless camera.