A simple, relatively low-tech improvement which could easily
be implemented *today* is the "gun-cam"; put a small camera
module under the barrel in the same fashion as one would
attach a laser/flashlight module. As (spy) cams can be made
very compact without losing too much picture quality, the setup
needn't be very bulky. You can even add some audio recording
equipment and perhaps even a data transmitter (burst
transmission?) so that even if the gun is seized or destroyed
by the bad guys, you still have the evidence. The camera
could be activated by means of a (pressure) pad on the grip
(a method used for certain laser aiming systems, though
I would personally prefer a system that reacts to pressure on the trigger for those). That way you only film when it counts. Further improvement could include a linked flashlight for when it's dark, and of course some safeties to prevent tampering with the data.
The advantage? Well, you basically make sure that there's a camera present at every shooting, filming from the position which counts most -- that of the shooter. That way a jury/ judge/commission can see for themselves whether a shooting was justified or not. At the very least every armed government official should have one, and of course it's highly recommended for civilian use too.
Is such a thing already on the market? If not, I hereby claim the idea... ;)