I agree with most of this. However, as a user who has years of first hand experience with various mirrorless cameras from different manufacturers, I'd say there are a couple of things missing which most mirrorless users will expect, or pass the camera by -
- First and foremost, Canon must get 4K video at least on par with, if not significantly better than, their main rivals. They need to start taking this seriously again, because the people buying a mirrorless camera expect it. This requires no R&D, they already have the technology. This should be an obvious slam-dunk. The sensor should be designed with in mind this from the ground up. No significant crop and at least 10-bit 4:2:2 internal with a decent bitrate. And low rolling shutter.
- As a component of the above, they need focus peaking, zebras and LUT previews as a minimum. Scopes, etc. would put them ahead of Sony's Full Frame offerings.
- Eye-AF. Almost every prosumer level mirrorless has had this for at least 2 generations now. For someone of Canon's stature, it needs to be at least as good as the Sony A9/A7riii. Sony is killing it with this. Canon's Dual Pixel AF is probably inherently better than Sony's overall system. It just needs to be fast enough to compete with the competition on a full-on mirrorless model, with the subject's eye priority included. This needs to be for video too, then the system will be the best in the world.
- Weather sealing. One of the big advantages Canon has for pros is that the L-series lenses are nearly all weather/dust sealed (the competition lag a little currently in this area) so the body must be too to take advantage of this.
- Same compatibility with flash and other accessories as the current top of the range EOS line. Again, this is a huge advantage to Canon. Some, like Fuji and Sony are still struggling to get full flash compatibility.
- Must be speedy and responsive all round.
- IBIS is starting to be almost a universal standard in mirrorless. Fuji are about to release it, Sony, Olympus and Panasonic have had it for years. Again, Canon should go in right from the start and not play catch up. And not digital IBIS, that's BS. It needs a sensor which actually moves, like the competition.
With the lens mount, I think it needs to be fully EF compatible with no compromises in performance or image quality. That said, I think it would be OK if it were somehow adapted - not in a clunky way, but fully integrated, weather sealed, etc. Or some kind of 'dual mount'. The reason being, a lot of mirrorless guys like the ability to adapt lenses. Personally, having been there/done that, I'm not so keen with a modern sensor for all sorts of reasons, but I can't deny a percentage of the market does love that aspect. Combine with the benefits of EVF like focus peaking or electronic split view (like Fuji) for actually using old MF lenses fully and you have a winner. That said, personally, I'd be happy with EF mount - like you say, the huge advantage is the massive selection of great lenses already there.
Size/weight - Somewhere between the M5 and 5D would be perfect, I think. Weight is more important than size I think. Style-wise, I think an upsized M5 would be just dandy - very nice design, somewhere between retro and modern, and I speak as a Fuji fan, but I think gradually the move will be away from full-on retro and towards a subtle blend. Canon already nailed this with the M5. Looks are important in the mirrorless market, like it or not.
And for those who wonder why do this at all - that's clearly the view of someone who has not extensively used a well-designed mirrorless camera. There are many, many reasons which is why the market has grown so fast and I have no doubt whatsoever is the future of ILC photography, but the one I now find it hard to live without is purely technical - I get sharper handheld pictures due to lack of mirror slap. Oh, and some photographers who like to use ND filters love them because they can still see what they're doing and even shoot portraits with flash and AF/see their subject.