Considering how close the rear element of a rangefinder lens can be to the sensor I would have thought that it would cause serious design problems with a full frame version. After all, the lenses are getting pretty far away from being telecentric as it is and this already causes problems with the M8.
Perhaps a new design of sensor with a flat surface would be needed, but I haven't heard of anybody planning that approach.
I would have thought the SLR design has proved itself much nore compatible with the requirements of digital.