I spent a few minutes today writing down my requirements for this device, as the end customer. Hopefully this will give people a better idea of what I'm thinking of, though it's still pretty generic.

The device SHALL

1. Mount to the handlebars of my bicycle
2. Provide position, heading, and velocity information to TBD accuracy
3. Be self contained and self powered
4. Record cycling trips with TBD resolution
5. Provide waypoint and track following information to TBD resolution
6. Be operable while riding
7. Have a display that is visible in direct daylight and total darkness
8. Provide a human interface for easy configuration and data entry without an external computer or keyboard

The device SHOULD

1. Utilize flight software framework(s) such as CoreFlight
2.  Utilize sensor fusion algorithms such as a Kalman filter
3.  Be resistant to environmental factors such as temperature extremes, dirt and moisture ingress
4.  Be removable from the bicycle without tools more complicated than a hex wrench
5.  Be extensible with new software and peripherals