Really - it is all in the viewfinder(s). The viewfinder is a present groundbreaking opportunity. Almost everything else is an evolution.
Imagine an LX-style body with various electronic and optical viewfinders. Imagine if the mount carried over when there was a body / model change. When you buy a camera the kit includes an FA-1 OVF with flash hotshoe - but you can opt for a body-only SKU and add optional VF's to suit you needs.
It shouldn't be any more difficult to seal than the battery door and the electrical connections shouldn't be any more complex than a tiny HDMI-style pinout.
And then there is this:
Originally posted by asahi man Better will be the Pentax Patent with optical finder and a digital overlay finder to have both in one optical system
Nikon has a similar older patent.
Best regards