A simple and cheap suggestion, tied to the stronger cockpit doors. Put a peephole in the door so the cockpit crew can see what's happening on the other side before they unlock/open the door.

Better yet, how about a closed-circuit TV system of some kind that has selectable views of the cockpit door area, the galley areas, and the cabin area. Again this would allow the cockpit crew a view into their aircraft without leaving the relative safety of the cockpit. This would also be nice if tied somehow into the data recorder systems, to help in the forensics after the fact so we might all learn how to continually improve flight safety.

BTW, another advantage of the decompression idea above would be that it would limit or eliminate the risk of explosive decompression if someone on the aircraft (bad guy, sky marshal, pilot) were to start shooting, set off an explosive device, etc. At least then the aircraft maintains it's structural integrity and can be landed safely.

-- jab