Audio loops have been suggested a lot. If you didn't know, they are tracks of audio that are played in the background on a loop, similarly to music, but are triggered by something such as the biome you are in instead of just playing randomly. An example of this would be crickets chirping at night, heavy wind sounds at the top of mountains or in the middle of the ocean, or even just birds singing. This would greatly add to game immersion. For the people who don't want it (which is hopefully not that many), I suggest it have its own option in the audio settings menu.
Something that would go along very well with this is better music placement. What do I mean? Well, imagine finding a structure and some kind of mysterious or even daunting music plays. Or, imagine it's night time, mobs everywhere, and the unused track "Excuse" by C418 starts playing. Or, imagine you finally defeat the Ender Dragon, and the other unused track "Equinoxe" plays, with it's similarities to both the Overworld and Nether music indicating that there is finally an Equinoxe–a balance–since the Ender Dragon has been defeated.
I think these two additions alone could make Minecraft a much more immersive and in-depth game than it is already. By the way, the idea for "Excuse" playing at nighttime wasn't mine; it was an idea brought up in the comment section on the YouTube video of it. Nevertheless, I strongly recommend that you listen to both of those music pieces, as they are very good. Another unused track that could be cool in-game is "Door" by C418. I think this music played back in Minecraft Alpha, but I know it doesn't play anymore. Videos: