I've watched all the episodes of Aviation Disasters. The device missing from cockpits is the video recorder. Many investigations revolve around questions like what is the pilot looking at, what the instruments show, even who the heck is in the pilot's seat. Video can confirm switch settings and instrument readouts (the voice recorder is also used to confirm if the warning horns sounded properly).

Instead, a lot of effort goes into trying to reconstruct this.

Just put a dang video camera in the cockpit.

I've also, for decades, advocated a camera that records flight operations at an airport. Just stick a couple in the tower pointed at the runways. Note that in the Concorde disaster, there was no video of the take-off. They had to rely on eyewitness testimony, which is notoriously unreliable. The only video came later from some passenger in a car who happened to have a video cam at hand. That accidental video proved valuable. (It's a horrifying video.)

I was surprised at all the vitriol opposition to pointing a camera at the runways. People confidently told me it would cost millions of dollars, and was completely infeasible. Jeez, anyone could buy a security camera that recorded on a loop for a few hundred. Every convenience store had them.