The most basic rear facing light would be a flashing (but not strobe) red light form a single, small source.
My 'push' is for something far more sophisticated, after all, the problem deserves it - the questionnaire and anecdotal evidence seems to show that accidents from the rear is a very common and serious problem.
I'm currently leaning towards a fairly large LED panel somehow woven into the fabric of a rider's jacket. The LEDs are individually addressable so that all manner of colours and images can be displayed. The most basic image would be a small LED cluster illuminated in the middle of the panel. As a car approaches, the cluster becomes increasingly larger, giving the car driver a visual impression that the cyclist is becoming larger - Its a psychological effect commonly used to 'nudge' people to do things, like slow down on roads. The image would further 'fool' the driver into thinking that the cyclist was becoming increasingly large at an increasing rate by becoming animated - zooming out from the middle in ever increasing rings or squares. I'll try and find an animation that describes this better