Include the Xvid and DivX codecs. I'd also include a selection of different skins for the GUI or at least the ability to change the color of the GUI using a color picker
This would enable users to matchup the skin with their windows setheme.
Please note:
Not sure whether these are contained within one of the other codes you listed. Sorry if I've made a useless suggestion.
XVID is supported via FFDShow.. : http://www.inmatrix.com/zplayer/formats/xvid.shtml
We do allow the additional installation of the XVID decoder but do not think it is necessary by default.