Thanks Gandalf and Edi... I wasn't really looking for slots which were not used, but more like existing units which are not used. Hence a mod could adjust their description, stats, etc., and provide new units.
Gandalf... I was also wondering what you thought of the AI Opponent Balance Mod I recently created. I thought you'd especially be interested.
http://forum.shrapnelgames.com/showthread.php?t=42679