A build challenge for Ben:
How many times have to gone to open a public toilet door (or similar - where no window is possible), only to have it open in your face - opened by the person on the other side you couldn't see. How about having distance sensors on each side of the door notifying an alert system on the opposite side of the door when someone is within opening range of the door. Maybe a simple LED traffic light - green = no one close by, orange = caution, someone there but not too close, red = danger, someone is very close to the door. This system could save some smashed faces or people falling down when the door they go to push open suddenly moves.