Earlier in this thread I made several points, one of which was a stronger, locking flight deck door would maybe be a way to go.
This has been met with several responses, some good points, some bad, some ridiculous.
As another of my points was there should be cameras in the pax cabin, pilots would be able to see if it was safe to exit the flight deck to investigate problems in the cabin (of a technical kind).
True, a possibility exists where many passengers may be injured or even killed if terrorists are on board an aircraft, but this is better than the terrorist being able to take control of the aircraft and use it as a weapon. This of course will mean the deaths of all souls on board as well as the loss of countless lives on the ground.
My final idea hasn't been commented on, perhaps because it sounds crazy or extreme. You must all excuse me if I am being naive or ridiculous, but it seems to me, once the flight deck is secure and sealed, with its own air supply (no recirculated air), the use of gas to 'knock out' all those in the pax cabins would prevent the hijackers from doing too much damage. Sure they would have time to do a certain amount of damage, but their time would be limited.
Once unconcious they would no longer pose a threat and the aircraft could land asap where the terrorists could be dealt with.
Even if this is not the right method to use, it seems to me that disabling the terrorists in some way BEFORE they can seize control of the aircraft has to be the way to go.