[ja30278]

Even simpler: Completely separate the cockpit from the passenger cabin.

Without the possibility to use the hijacked plane to take hostages to a friendly destination, or to use the plane as a missle, it's no more attractive as a target than any other public space.

[ceejayoz]

This is already essentially in place, as https://en.wikipedia.org/wiki/Germanwings_Flight_9525 demonstrated in an unfortunate manner.

[pavel_lishin]

Simpler, but not easier - you're talking about redesigning basically every airplane across the world, including putting in another bathroom and a small kitchen area. That adds weight, cost and complexity, and may actually make things more dangerous for pilots if one happens to have a medical emergency.