I'm not a motorbike rider (although I have ridden one as a teen and did spend several years working in a big motorbike shop/warehouse), but I do a lot of cycling and would really love to get some kind of separate HUD. I'm thinking of something like Google Glass, but with swappable lenses (e.g. clear, high contrast yellow, shaded) that could link up with other instrumentation via bluetooth.
It'd be great to have a HUD displaying a rear view camera, GPS info and other cycling specific info like cadence, heart rate and speed. If the front and rear cameras also stored video onto micro-sd cards, there'd be great evidence for use in any traffic incidents.
If it's in a separate form factor, then you could wear them under a motorbike helmet as well as wear them on a cycle which makes more sense if you do want to replace your helmet. I'm sure there's a lot of similar functionality that you'd want for both modes of transport.