I don’t know if the world really needs higher fidelity audio. I doubt it.
But it would be useful to be able to choose a version of a file that was mastered to match your listening gear. One version might be mastered for the classic low end white earbuds that come with an iPod. The other would be mastered for good speakers in a room with reasonable acoustics.
This would be different than EQ settings customized for the listening device. EQ is only one of many tools a mastering engineer uses. For example you might want more compression for iPod earbuds, because quiet sounds are inaudible and loud ones distort. Or you might want less reverb on earbuds, because it makes it harder to distinguish a fast series of staccato notes.
Choosing these settings isn’t really a job for an automated system. A human needs to drive it. That’s why people find the money to pay mastering engineers. So you can’t just build in a standard switch in the player – there would need to be two files to choose between.
I’m imagining an audio file format that allows you to switch between different masters depending on context. They would both be in the same file. It would be basically a multitrack file where the different track types were well known in advance – like “hifi” and “lofi.”