If canon put decent AF on the EOS-M it would be perfect for me. The point of the mirrorless is that it is so small. When you add things like OVF and full frame the whole system just gets bigger defeating the whole purpose. I have a 5D3 and if I want shots in the dark I will bring it with me. I am perfectly fine composing and shooting using the back LCD, I would like it as small as possible with Canon 1.6x crop IQ.
The only other would-be-nice change would be to make it look retro like some of the Fuji and Olympus cameras coming out but that is not a deal breaker at all.
I think your on the money except for the retro bit - I couldn't really care whether it looks contemporary or retro.
The APS-C sensor makes it possible to reduce the flange distance. With full frame, there is not much scope for doing that, without funky micro-lens arrangements, which to my understanding Leica has patented. Canon already has problems with the oblique angle of incident light from the 85mm f/1.2. Bringing the rear element any closer to a full frame sensor would be a big technical challenge. As a result, I believe it will be difficult to make a full frame mirrorless camera which is much smaller than a DSLR. In addition, once you are using full frame lenses, you need a larger body in order to make for a balanced system which can be held comfortably.
APS-C quality is, to my mind, good enough for a small camera. There are people out there producing absolutely stunning images with APS-C sensors.
I would like a viewfinder - as it gives a better ability to brace the camera. I would also like a wired shutter release. Once those are available, together with a newer APS-C sensor, I may be prepared to buy an EOS-M.