As I've mentioned somewhere before, it would be trivial to make a "smart frameline" finder for the next Leica rangefinder camera. Throw out the metal mask, the arm that moves it, and the illuminating window. Install a small LCD display, monochrome, backlit. It displays the correct framelines. You just need to read the lens lug, and a very high resolution shaft encoder on the rangefinder arm. That can automatically create the right frameline, including parallax and frame size compensation. It can also use the six-bit code (if available) to display just the one correct frameline, rather than both ones associated with that lens lug cut.
There could even be a menu to control how conservative to make the framelines (90% to 110%), how thick to make them, solid or corners only. They could also display the lens focal length at the bottom. Heck, they can display the shutter speed there too, along with the exposure readout.
Of course, the menu would also allow selecting an arbitrary focal length, just in case you're using a Hektor 73mm lens, a Summarex 85mm, or a Nikkor 105mm, none of which have framelines in any M camera today.
You also get framelines that don't depend on ambient light for illumination. This may also allow some changes in the optical path of the finder.
The frameline illumination level can also be set by the external photocell, since you obviously do want to dim them indoors.