The dream fitness tracker doesn't need a phone. Sure, taking one with you on a run is often a good idea if you might end up far away from home, but with average screen sizes getting larger by the year they're hardly becoming more convenient.
However, only a few fitness trackers will work as standalone music players. We're not expecting a 3.5mm jack on the thing, but the ability to hook up with a pair of wireless headphones and, say, 8GB of internal storage would be neat.
There's no major technical issue here besides the little bit of space needed for the extra storage chip. Few companies bung it in because it's going to raise the price for what is still a side order many people might not use.
Music-ready trackers of today include the TomTom Spark, MiCoach Smart Run and the Lenco Sportswatch (we can't vouch for the quality of that one, though).
Any Android Wear watch will be able to do this too. Not that any of them tick many of our other wishlist boxes, mind.