Flatscreen TV’s are an amazing space-saver. So why don’t other electronics follow suit? With the same flat orientation of the TV applied to other electronics (like DVD players, stereos, and game systems), you could have an entertainment unit as deep as 6 inches, or about the width of a DVD case.
Sure you might have to fasten things down to keep everything firmly in place, but it’s a small price to pay to gain additional room. The reclaimed real estate would be beneficial to those living in a smaller space.
As for the component electronics themselves, a few design changes would be needed, like the DVD slot (see above). and a way to conceal the wires (something like this flat PS4 concept).
Dave's Geeky Ideas 
Well, you can get quite high end hifi mini systems that can be used to plug DVD sources into, etc… some even feature(d) DVD playback. I daresay some have been made that have Blu-ray functionality.
Only a little bit slimmer than a typical modern video player or separate amp, granted, but it’s still something. They’re often not much deeper than a DVD case, certainly. And some lower end stereos exist that are of this kind of design (e.g. discs go in vertically). It’d be worth a look to find out…
If you can get it all slim enough, and have it so e.g. the cables attach to sockets that point straight down rather than out (or you get cables with right-angle plugs), you may even want to consider having the DVDs racked diagonally (turned 45 degrees or more from the perspective of looking straight down at the top of the box) on the shelf, the way some parking lots and hire store shelves are arranged.