One issue that has yet to be fully explained is how pilotless aircraft will integrate into the Air Traffic Management system. Will they operate alongside current "conventional" aircraft, or will they have their own airspace? Will air traffic control be able to contact these pilotless aircraft? Will they be following a pre-defined flight plan or will they be remotely piloted?
There is a third option; they could be autonomous, making routing decisions based on the situation around them. The input instructions are at the high level only - "Your schedule is XX, your destination is YY. You must comply with the regulations and give due account to weather, proximity alerts, onbboard status alerts, ATM instructions/restrictions/updates etc, your diversion criteria are ZZ and your available diversions are AA or BB, and you must conduct the flight with the lowest possible fuel consumption commensurate with these requirements". The technology to do this already exists ATM comms would be in the form of a datalink signal rather than verbal, but the controller would just be pressing message buttons rather than talking.