While arming the pilots might be a nice idea, remember that they are facing forward and turning around for them is a rather awkward manuever. They might be able to do somethign if they were aware of something going on but not, for example, if a hijacker would come flying through the door.

Maybe ifthe 3 man, or 4 or 5 man (navigator, radio operator) cockpits are brought back, it'll be practical but not otherwise.

As for armoured doors, not practical for decompressions reasons as other have said. And what would happen if the flight crew got incapacitated? A remake of the Payne Stewart thing with a different twist.