Dogs of War in Warmaster
I always thought this was an excellent idea of GWs for a mercenary army and units to hire, hopefully this will be expanded upon in Warmaster.
There are some excellent pictures of a Dogs of War army in the Warmaster 2003 Annual.
Regiments of Renown in Warmaster
There are a variety of Regiments of Renown in the Warhammer world, both old and new, from Bugmans to Drong, there are many different types to use in Warmaster. Some Regiments of Renown can be used as mercenaries others will only be found in their race's armies. For example, Bugman's Dwarf Rangers will only fight in Dwarven armies.