I always chuckle when I see folk wanting full frame mirrorless. They just haven't got the concept. Big sensor means big lenses. big. big. Not small.
Isn't the opposite one of the reason why people like the leica system?
The OM-Zuiko lenses were similarly sized. None of the Leica M-flange lenses offer autofocus which reduces diameter enormously. Secondly, the lens register is 27.80 mm compared with 18 for EOS-M.
Third, the aperture in an EOS-M flange is too small to permit a 24x35 frame to be fully illuminated by a lens with a larger register.
My third point is critical - it forces the lens' exit pupil to lie fairly close to the flange. If you were to use a 24x36 sensor, together with an EOS-M flange, light from the centre of the lens would reach the corners with an angle of incidence of (something like 45-50 degrees. This corresponds to a light cone of about f/0.5 - f/0.4. It's already well established that the sensors have difficulty detecting light from a f/1.4 light cone, so corner pixels wouldn't actually detect any light.
Among my cameras, I own a Fuji X-E1. In all seriousness, it gives my 5D2 a run for it's money. This has lead me to rethink the real value of large sensors.