To be honest I have NO idea how hard this would be to produce or how expensive but it would be cool to see some form of Eye tracking sensor I’m almost certain the technology itself exists and I think it would fit in well with some of the other biometric sensors you guys offer like the myoware muscle sensors the neurosky mindwave mobile+ and the pulse sensor.
It doesn’t require a specific sensor.
You can Google “Eye Tracking Sensor” - there are many (411,000) results.
Sparkfun had this article in 2011: