The RF coupling can be dropped if Leica decides to use a hybrid optical VF. The focus would be read from the sensor, as cameras do with EVFs, and a digitally simulated rangefinder patch could be used in the optical VF.
No more RF drifting, no more focus shifts from aperture changes, no more needing to calibrate lenses to the camera etc. But you still have the optical finder.
Frame line linkage can also be removed as the frame lines can be digitally displayed as a result of the camera recognizing what lens is mounted via the 6 bit coding.
Heck, even the LCD panel on the back can be dropped as with the new MD. Almost everyone carries a smartphone with them, and so if exposure or focus or composition needs to be checked, the camera can use WIFI to connect to the phone to seamlessly bring this up if needed. You could even use a smart watch!
There are so many possibilities for the M in the future, it just depends on in which direction Leica decides to go.