Another suggestion:

Give the pilots a) a better reinforced door and b) the ability to depressurize the cabin without the masks dropping. This would permit the pilots the ability to knock everyone out if necessary.

A possible scenario: hijacker bent on self- (and others-) destruction grabs someone in the back, threatening to kill them if the pilot does not turn over control of the a/c. F/A rings a panic button. Pilots grab O2 masks and push a couple of buttons, depressurizing the cabin and within 30-40 seconds knocking everyone out. Of course this would have to be above what, 25,000 feet?

Pilots then head for the nearest a/p. Authorities are waiting to sort the terrorist(s) out before they regain consciousness.

The terrorists can't get into the cockpit to thwart this procedure because it (cockpit) is secure. The only way around this would be for the hijackers to smuggle O2 and masks onboard. Not likely.

Risks: a) smothering everyone on board if depressurization at too high an altitude. Keep the cabin at a pressure able to support life but low enough to accomplish the knockout. b) terrorists initiate takeover too low for depressurization to work. Pilots declare an emergency and quickly climb (a couple of minutes at most?) to an altitude where this would work.

If the terrorists want to use the plane as a weapon, turn it back against them.

Mike