so obviously what ever they are doing is already decided and in motion. But just to speculate...
One of the key problems is that chaos corruption doesn't do enough to deter factions from resetting razed settlements.
Vampire corruption works because your actively able to increase corruption with buildings so your nonboring zones are getting infected. chaos doesn't have that. as soon as your army moves your basically losing the corruption.
so my proposal is what if chaos corruption spawns more chaos armies. Like you get a zone to 100 chaos corruption and it spawns another chaos army. if they go the mono god route they can have a diplomacy system like depending on your relation to a god affects how the enemy army sees you. They could be new hordes with the opportunity to confederate them later. If you have a bad relationship with there God they will just be another enemy.
Basically its the evil version of the empire. You coddle the other war bands to you eventually take up the matter of Ever chosen and then you unite all the warbands under your rule. the extra warbands give a larger distraction in the area your fighting thus preventing the factions to bee lining to the ruins. it also encourages build up. you get an area and hang around it for a bit to get more potential allies, like having chaos preacher gathering converts.