Google glass, but WITHOUT ANY FEATURES.

It's just a rear-view mirror, via the camera.

(No phone calls, internet, or any of that junk.)

Just a rear-facing camera (stuck onto the helmet, like a Sena, or maybe built in) which provides video feed directly into the driver's left eye, using the "google glass" style of prism. There could be patent issues (Google have a patent for their idea) but I think this idea is fundamentally different, and has nothing to do with the internet or social media, so it stands as a legitimate application for optical technology.

If you haven't seen google glass, it looks like this:

http://wordlesstech.com/wp-content/uploads/2013/04/How-Google-Glass-works-1.jpg

Motorcycles badly need an equivalent of the rear view mirror we have in cars. This could improve motorcycle safety.