I like the idea, although unfortunately I'm not sure how to propose it in a way that doesn't potentially insult the awesome work of Uematsu on this game. It's just that MMOs have an even longer playtime than regular RPGs, and if you hear it enough times you'd get tired of even the best songs, especially the ones in the cities where many people are most of the time. I'm nowhere near that point yet, but it will happen eventually--I quite liked FFXI's soundtrack but after a few years I found myself listening to other music in the background and muting the game's music, simply because I'd heard it so many times.

As far as implementation, I think Unreal Tournament 2004's good simple playlist manager that played MP3s and .oggs would transfer quite well to this game. I'm not sure how the sound is encoded in this game, though, so I don't how how many man-hours it would take to do this, but it'd be quite nice in the future.