So, this will be a fairly niche suggestion, but here goes:
I have asymmetric hearing loss (loss in both ears but much worse on one side than the other and a different profile for each ear) so I find headphones very useful. But what I would love to see is the ability to adjust the sound at separate frequencies. I don’t mean adjust the individual cans (although that would be optimal!) I just mean the ability to adjust the frequencies to more closely compensate for my hearing profile, via a settings app. The kind of controls I would like would similar to the Ears Audio Toolkit plugin for Chrome, which I find invaluable for making Netflix etc much easier to decipher.
I know that this might be difficult to implement, but it’s a feature I, for one, would love to see.