Is there any reason in principle that large aircraft couldn't be converted to be controlled remotely? Suppose the pilot or pilots of a plane become incapacitated for whatever reason. At the moment, there is nothing that can be done from the outside to stop it other than to shoot it down (am I right?).

Would it in theory be possible that planes have a 'safe landing' program built in that, at a signal from the ground takes over control? It then does everything necessary to land correctly and automatically at a nearby airport. 

I realise I am making a lot of assumptions here. My main question is not about the logistics  of what would have to happen on the ground, nor is it about whether a new generation of terrorists could hack into such a system for their own purposes. It is about how easily current autopilots could be adapted to have a 'land safely' option. What modifications would the manufacturers have to install within the plane itself that would make this possible?

Summary


Is it true that currently, the only way to control a runaway aircraft is to shoot it down?
In theory could an existing autopilot have a simple interface that is retrofitted such that a single signal from the ground  sets it in 'return to base' mode and produces a safe landing?
Yes for reasons of hacking prevention. There is no way to be absolutely sure any interface can't be hacked to crash the plane remotely into a suburb.
Yes autopilots can follow a track of navigation marks to an airfield and can auto-land using the ILS.


But again there is no way to ensure only signals from people who have the best intentions with the plane can activate that mode.