Here's some small suggestions to make Nurgle more flavorful

1. Plaguebearers are unique in that they are made from the souls of those who die of Nurgle's Rot rather than being generated by Nurgle himself. I'd represent that in game by reducing the trickle of Plaguebearers you get from the Fleshpoppy building and have several added to the pool whenever a disease has finished ravaging a settlement. One Plaguebearer for every settlement tier up to the third one and one Exalted Plaguebearer for T4 and T5. So a T5 settlement would yield three plaguebearers and two exalted Plaguebearers for the pool
2. Rot Flies are the result of Beasts of Nurgle getting banished from the mortal realm and developing seething resentment over it. So any time a Beast of Nurgle is slain, one unit of Rot Flies should be added to the pool to represent that
3. The Deathshead Grenades hurled by the Plague Drones are made from the skulls of people that get eaten by the Rot Fly. So I suggest that Plague Drones should have an ammo replenishment effect in combat like the Skull Cannon