Most people automatically think that old consoles like the Master System, Nintendo, MegaDrive, etc, have an aspect ratio of 4:3 simply because that is what the TV has. However few consoles utilize the whole screen and have borders depending upon which region they are in.
I was looking at a lot of videos of real Sega Master Systems and noticed that the way they were displayed on the TV was different to how I was displaying it in my emulator. I always knew the real systems had borders but I never paid much attention to it thinking it was just a "simple color" thing.
However the borders force the aspect ratio to be different than is typically thought. A PAL SMS is almost a widescreen aspect ratio. Take a look at the following image. :-
Notice how the actual content is taking up a much smaller amount of the 4:3 screen real estate? Of course some of that border is "chewed up" by different TVs and how they handle overscan. Most emulators seem to take the game image and force it to 4:3 aspect ratio, which if you look above you would realize will distort the image. I was doing this in RetroCopy but I also have an option to specify whatever aspect ratio you want. I originally added this feature simply because TVs have different aspect ratios themselves, I somewhat forgot that the source content wasn't 4:3.
Not only that but I noticed that some games used the border as an extension of the display. California Games for instance :-
Whilst it may seem a bit simple, the games which utilize the border region actually make the games themselves more visually appealing. Alex Kidd in Shinobi World flashes the border region during the introduction and it was much more appealing on the real system with the borders than in my emulator which didn't have them. The shot above is taken from RetroCopy which now has border emulation.
The borders on the SMS weren't really taken advantage of, you should be able to at least change the border color every line to get a stripey type pattern. One thing which will need further research is how much you can utilize the border region. It may be possible to update the color about 5-10 times per line, which would give you even more freedom to display different things there.