I know this might be a lofty request, but I think a one-time chance to change to another biped race when going from F2P Human to a pay account would be awesome.
Even if it was just offered as a pay service, I think it could be nice. I've not gotten deep enough into the game to see what kind of problems it could cause. Obviously changing to and from Dragon wouldn't work. Just the bipeds...
EDIT: After posting this, I realized it might fit better in the Suggestions forum. Sorry for putting it in the wrong place!