the mini player is my player of choice when doing everything else except just listening to music or updating/editing my library.
while I won't go as far as lnminente and say that it needs to be fully bitmap skinnable, I do believe that more skinning attributes need to be available as sometimes the colour combinations that are pulled from the main skin are not optimal. Often leading to a compromise in what you want to achieve in the main panel, but still keep it legible in the mini-player. An option to have the player controls located LH/RH would be useful when using those bitmap skins that do have RH controls.
a couple of mock-ups for a metro option in the horizontal layoutwith a spectro, the hidden options available on mouse-over
and for a vertical "widget style" layoutclosed - classic style

this would be basic view with least amount of info
open - metro style

each section would be optionally displayed
wavebar not progress bar
if "progress bar at bottom" option was used - it would move that panel to just above the open/close bar
full controls option adds bar under player controls
artwork/artist display option
spectro option
if "show ratings"/"show love" options are enabled - they are in open/close bar
not necessary to have dividing lines, background could be blended. Lines are just to show how options could be easily slotted in or out.
FWIW - that's my suggestions