A rear view camera coupled up to a small head mounted display, like the ones that attach to the side of a pair of glasses, would work well. It's just the thing I need on the 'bent, as looking behind is difficult and I have to rely heavily on the mirror.
To add to the functionality, once the display is there it would be relatively straightforward to add all the bike computer and battery power consumption data to the display, removing the need for any clutter on the handlebars.
For those into long distance touring, a GPS-driven moving map might be implemented the same way.
Jeremy