I have often thought of this same concept.
Make a TLR with the top half and the 'viewfinder' as electronic and the bottom half for film. The waist level viewfinder would double as the electronic review screen.
Have a two-stage shutter button that lets you choose one or the other or both. It would be easy to build in a double-exposure warning/prevention for the film stage.
Construction should be simple using a fixed focal length. They would just have to work out the focusing linkage between the two stages.
Leave auto focus out of it and keep it very, very simple.