What if cockpit door lock was controlled from ground?
For this to work, we would have to depend on everyone on the ground to have the fortitude to tell the pilots, "Sorry about the passnegers they're killing, but we can't open the door." And for the pilots not to lie to ground control in the face of threats.
But it seems like a relatively simple solution that would raise the bar quite a bit. Wouldn't require recertification of the airframe. Could probably be tested and approved pretty quick. Maybe include a "panic button" that unlocks the door but engages the autopilot in a way that can't be disabled except from the ground.
