I really hope that mercenary groups are a big part of playing some factions. The Dogs of War have a lot of character and could be an interesting way of showing interesting mechanics in unique type of units.
You got the Pirazzo's Lost Legion
, guys who were poor and wanted richness and became famous for fighting and surviving in harsh environments. They even helped the Lizardmen fighting the undead. So they could have some kind bonus when fighting in some scenarios.
There's The Birdmen of Catrazza
, guys who strap themselves in a bird suit and fly over the battlefield shooting with a crossbow.
Also a great way to introduce Ogres which are also famous for doing a lot of mercenary work.
Just check out this wiki: http://warhammerfantasy.wikia.com/wiki/Dogs_of_War
Would be really interested to know if CA has any plans for this subject.
(note: Somebody less lazy than I could elaborate more about the Dogs of War for those who are not aware of who they are!