Today, fewer and fewer photos are printed. We usually see them on screen. And more and more commonly, we see them on a widescreen monitor. 16:9 screens are quite common as are 16:10. You can hardly find a 4:3 screen any more, though that is the aspect ratio of most P&S cameras. Most SLRs are 3:2, which still doesn’t fit on the widescreen monitor.
So there should be a standard tag to put in photos saying, “It’s OK to crop this photo to fill aspect ratio X:Y.” Then display programs could know to do this, instead of putting black bars at the center. Since all photos exceed the resolution of the screen by a large margin these days, there is no loss of detail to do this, in fact there is probably a gain.
One could apply this tag (or perhaps its converse, one saying, “please display the entirety of this photo without crop”) in a photo organizer program of course. It could also be applied by cameras. To do this, the camera might display a dim outline of a widescreen aspect ratio, so you can compose the shot to fit in that. Many people might decide to do this as the default, and push a button when they need the whole field of view and want to set a “don’t crop” flag. Of course you can fix this after the fact.
Should sensors just go widescreen? Probably not. The lens produces a circular image, so more square aspect ratios make sense. A widescreen sensor would be too narrow in portrait mode. In fact, there’s an argument that as sensors get cheaper, they should go circular and then the user can decide after the fact if they want landscape, portrait or some particular aspect ratio in either.
The simplest way to start this plan would be to add a “crop top/bottom to fit width” option to photo viewers. And to add a “flag this picture to not do that” command to the photo viewer. A quick run through the slideshow, tagging the few photos that can’t be expanded to fill the screen, would prepare the slideshow for showing to others, or it could be done right during the show.