I would like to see mp3 players with a crossfeed circuit to simulate speaker listening. Most recordings were made with the intention of being played back through speakers, and each ear hearing both the right and left channels but with a lower volume and a slight delay from the further speaker. This enhances the soundstage. An adjustable crossfeed circuit would mix a bit of the right channel into the left, and the left into the right with a slight delay. Having control of the intensity of the effect and the delay would enable fine tuning of the effect.
With the rise of mp3 players, I am surprised that we haven’t seen a large number of binaural recordings released. Binaural recordings are made using a dummy head, with mics placed in the ears. Binaural recordings are optimized for playback through headphones. Perhaps a music label could be bold, and sell a traditionally recorded version of the music together with a binaural version.