Try to remember that if something is not used in flight, then it should not be fitted to an aircraft - just about everything else can be located on the ground. Concorde didn't even have an APU because it was never going to be used in flight...
How about a very relevant idea:
A GPS marker which is ejected from an aircraft as it hits / just before it hits the sea.
That way the impact site could be easily found and we would not now be searching the sea trying to find a missing aircraft.
This would also have helped in AF447.
The marker may or may not have flight recorder data stored in it.