How come commercial aircraft don't have some sort of weight sensors built into the landing gear?
It'd provide a way of measuring weight and balance directly... and providing a check on the crew's calculations. And, if the measurement comes up way too light, and it's not an obvious sensor failure? Maybe take a dipstick to the tanks.
I'm thinking strain gauges bonded to struts, or pressure transducers on the shock absorbers: something that can (after calibration) give reasonably accurate readings, but doesn't involve adding a new point of mechanical failure.
Also: how about video cameras with views of the engines? Couple decades back, I was involved in estimating a deluxe version of such a system (high-res, high-frame-rate cameras to watch for FOD during takeoff and landing, plus storage for the video); given the requirements for that application, we concluded that it was wildly impractical using the technology of the day... but, with today's technology, and if the requirement is just to allow the crew to see if an engine is on fire, or missing, or otherwise grossly wrong - and to be certain which engine is out of whack - it seems nearly trivial, and potentially useful.