The Leica M12 will be 3mm smaller compared to the M11.
I am still getting conflicting reports on the viewfinder: we may get a new hybrid viewfinder like the Fuji X100 or maybe even a 100% electronic viewfinder.
The ISO dial on the top left could be completely removed.
A new dial could be introduced behind the shutter button that can be controlled with the thumb.
Large 3.9″ screen on the back (the M11 has a 2.95″ screen).
The large screen means also removing most (all?) buttons from the back of the camera – maybe we will get a touchscreen interface similar to the Leica TL:
There is also a new Leica M12 trademark, but it seems that the application is denied: