I use the iTunes quite much because it helps me organize the music nicely and because Winamp is not available for Mac OS X.
But most of the current players have the following problems:
1. Songs are duplicated.
2. If songs are relocated all your albums and tags are gone for a toss.
So, the basic principle I am planning to have is Accoustic Fingerprinting [http://en.wikipedia.org/wiki/Acoustic_fingerprint] as the core.
The moment a file is added into the system it would generate an id for this which will be used as the primary key for everything like play lists.
The same concept can be extended further to provide downloading of songs if law permits.
The SDK for audio fingerprinting is available at: http://musicbrainz.org/doc/libmusicbrainz
The UI can be written using Mozilla's UI components as these are easy to use and app would work across all plateforms.