I don't know about you but, bedrock fog (also known as void fog), was one of my favorite features in Minecraft before it was removed. I know a lot of people didn't like it though, so I think it would be a good idea to add it back and put an option to enable it or not.
Supposedly, they removed it in 1.8 because of "performance issues" but I don't believe that for an instant since it had no noticeable impact in 1.6.4*. Well, aside from the particles, but they can re-add it without them, or with a more reasonable number (they would immediately max out the particle limit and cause FPS to be halved).
*I've measured the performance impact of void fog; this was measured over 1000 frames after things had stabilized:
Took 3088185.0 nanoseconds per frame to render particles.
Took 5885.454 nanoseconds per frame to execute void fog code.
As you can see, 99.8% of the time taken to render void fog was spent on particles - calculating the fog itself is all but unnoticeable (if these were all the game did in a frame the frame rate would be 323.2 FPS for both, 323.8 FPS for particles only, and 169,910 FPS for fog only. Given that overall FPS halved when particles were enabled this means that half the entire time spent rendering was spent on particles alone).
Not that I'd use it if it were readded and I played in a current version; I've used Optifine to disable it ever since I started playing, later on permanently disabling it in my mods (not even Optifine can re-enable it since I disabled it in code that it doesn't modify, and how do you appreciate the size of caves if you can't even see more than a few blocks?).