A Big Red Button
Previous posters have suggested the equivalent of a big red emergency button, with , as one poster put it, minimal checks to make sure it was only activated on the ground.
Looking at that not from the pilots perspective but as someone who has to design and implement such computerised control systems it would seem to be a nearly impossible task. Even with today's advances in computing, you have to specify things in black and white, go/nogo, binary type decision trees at the lowest level.
So what criteria could you use to let the automation decide you were on the ground?
RA? not accurate enough. Wheel spin up to a certain speed? No good if aquaplaning. MLG compressed? Both or one? That's why AB does the partial spoiler extension to dump lift and get both MLG compressed - but that is itself switched out if one of the TLs is not at idle. Here we go round the loop again.
Wht I am trying to say is that a lot of thought already seems to have been put in to the area of deciding "on the ground", and it may turn out to be not good enough yet. But i don't see how a new emergency off system would be anything but a) a duplicate of parts of the logic already involved in the decision making process, and b) more complex than that which already exists.
Also remember that any hardware component can develop a fault. Both solid state and mechanical devices can wear out/breakdown.