Was reading Gotrek and Felix "Skavenslayer" last night, and the chapter on Pestilens gave me an idea.
I mean 1 is the obvious addition of a "workshop" for them, but this is more of a superweapon to add with it; "The Cauldron of 1000 Poxes". I also think it would likely be a really easy thing to add.
1: the really easy method would be just a "Plague" spell with maybe a significantly larger AoE and damage
2: a more interesting method, make it a large permanent malus you can drop on the battle map (at least till the battle is over) that does DoT to stuff inside it as well as maybe imbueing poison and contaminated (perhaps except for Plague Monks, Priests PlagueClaw Crews and also Undead). This would provide the first real area denial tool which would be cool imo.
Either way it should be upgradable in the workshop, like the doomrocket for more damage, effects etc.