Working on an MP3 player alternative myself (alas no OGG output, if you can find a good library for it I might add it later) which already does things the official one doesn't (for example actually plays all mp3s I've got properly regardless of bitrate, and does seeking back/forward which is important for talking books), and will include an equaliser, possibly normalisation, definitely quality playlist handling and may also make it skinnable if I can really be arsed.
Even planning on adding a power save feature so it puts the GP32 to sleep if left idle.
Got plenty of cpu time free at the moment, but then it doesn't do much graphical stuff (text interface at present) so I may even be able to clock it lower than the 67Mhz default, but whatever I add I have no intention of taking it higher.