The major effective security change was the acknowledgement that no passengers would be allowed in the cockpit. This was realized, too late, by the 4th plane passengers on 9/11. The FAA should have realized it years before given the terrible outcomes where passengers had been allowed access to the controls by the crew and drunk people had caused trouble by forcing their way in.
Given the difficulty passengers had in breaching the existing door on 9/11, it is clear that the existing doors were sufficient. Add in that passengers also generally know the problems they will face if anyone with ill intent gains access to the cockpit and that would seem to eliminate the majority of the problems.
Concepts about shooting and killing the pilots are far fetched given that shooting out a window gives a clear shot at disabling an engine while providing a major distraction for the crew, which is nearly as effective and more obvious in result. Maybe add bullet resistance to cockpit seat backs?
I think the threat of converting a passenger aircraft into a suicide-guided cruise missile is over. Concentrate on fixing maintainer induced problems and crew upset training and the other, repeated sources of crashes.
If they want more security theater, maybe a curtain similarly retained the way cargo nets are, but opaque, so that no one can see the cockpit door and no one can simply rush the opening.