Some notes on what an audio player (somewhat targeted at dancers / DJs) should support:
This is a side project I'd like to try my own hand at some time. If you want ultimate, I've got a few requirements from an interesting angle for you: I'm both a dancer, and in some regards a DJ. Specifically, I tend to play music for dancers. iTunes absolutely falls down here.
- Replay gain: Replay gain is an amazing feature, and once you have it, you never want to be without it.
- Do not trust duration tags. They lie. A lot of media players (Foobar2000 included!) trust these. Or assume MP3s are CBR, and that the remainder is the same bit-rate as the rest. It greatly affects usability too, when the player thinks a 3 minute song is 30 minutes, and scales the seeking bar to that point.
- Robust codec support ogg and flac are a must, as they're the two best formats out there. (ogg beats the pants off mp3, and it beats mp4 by virtue of no patents; flac is the only good lossless codec I know of, which is important¹.)