Thoughts about human civilizations there: the Southlands are large and unexplored, which would be the perfect place to put new, hidden armies. An African-influenced nation in the Southlands can draw inspiration from Great Zimbabwe, maybe a series of fortress-cities deep in the jungle that protects the people from the dangers there. They have encounters with Lizardmen and fight the greenskins quite regularly.
To connect it to the rest of the Old World, they might have had skirmishes with the Tomb Kings and ancient trade with the Araby (think Mansa Musa), and there are myths and legends in the Empire about them even being Sigmarites! Consider the legend of Prester John or the Queen of Sheba, and King Solomon's Mines.
It might end up a mishmash of different SubSaharan African civilizations from history, (as opposed to say if Cathay and Nippon were armies, western audiences would be able to distinguish between them more easily), but it would be cool because it would give representation to cultures that don't show up in most traditional fantasy, except maybe Guild Wars Nightfall.
In addition to this, Southlanders have okapi cavalry and fight "savage" versions of humans, elves, dwarves, etc. who have been driven mad by the all-encroaching jungle. Hyperintelligent gorillas, too.
Southlands (or Lustria) critters and ideas
in WFRP Gamemasters
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