I was doing some reading on equal loudness curves today and the article mentioned something that got me thinking.
Loudness curves are greatly influenced by the ridges on our ears. Sound we hear every day usually comes at us from some angle other than perpendicular to the sides of our head, bounces off the ear ridges, and then enters the ear. Those ear ridges accentuate some frequencies and attenuate others. Headphones fire right down the canal bypassing that. Hence we are dealing with an entirely different head transfer function when mixing on headphones messing up the chance that it will sound correct out in the real world.
So...What if somebody made headphones with large ear cups where the drivers are parallel to our face firing at the front of the ears instead of down into the canal? They would have to operate at louder levels than regular headphones since the driver would be farther from our ear canal, but they would get the ear ridges back into the game and we'd be working with our natural loudness curve. Would be interesting to try.