If you still need an Idea how about a relational GPS. Instead of using the satelites to tell you where you are you can use a differing transmitter to tell you the proximity of your location to that of another location.
So lets say that cars come with these devices. Then the receiver can tell you how close you are to an oncoming vehicle. Another use would be hunting. Bow hunting especially. You hit your target and it runs off the "GPS" in the arrow head can determine the location from it to the receiver (hardware or software).
So while you can use the GPS to tell you where you are you can use the proximity locator to tell you how close you are, whether it is left, right, in front, behind and how high up it is.
The other is a remote transmitter (much like currently exists in the military for the drones) to function on a "dead" radio/hijacking of an airplane where the tower would have complete control of the air plane and the GPS would fly it to the closest airport. The stick inside the cockpit would be unresponsive and can only be overridden with a safety code.