If you're adding that, you might as well add a "sync with Wizard of Oz" function.
Honestly, as long as you can sort/filter on several categories and make your own playlists, you're good. I don't think people still want those all-in-one programs for their music player. They want a lightweight program that plays music with a clean interface.
If you're just adding functions for the sake of adding functions, you could try adding some effects (echo, reverb, super-bass, etc). I'm not entirely sure what you can do with mp3 format files though. I doubt you have much control over the content.
An easier thing might be to loop a part of a song. I often find myself having to pause, search, click and play because no music player I know has this function. It's very handy if you really like a certain part, e.g. when trying to learn some badass riff.
They want a lightweight program that plays music with a clean interface.
One of my biggest conceptual challenges is to figure out how to keep the interface clean and make the program as easy to understand and use as possible as I continue to add more functionality.
I'm not necessarily going to add all of these features, I'm just considering them.