Seems to me that having a simple GPS unit that transmitted the aircraft's position every 10 minutes would go a long way toward finding a missing plane. It would certainly narrow the area in the event of a search.
If you want to improve on that, allow ATC the ability to send a signal to the GPS unit that would cause it immediately to respond with its current location. In the event of a mayday/hijack code/9-11 type incident, the ground would know exactly where the plane was.