Chaos Wastes go without saying. It’s disappointing there’s no Daemons to begin with. But at the very very least you could add some ambient Nurglings in stages where Nurgle is present.
Convocation of Decay ends with a ritual summoning a Great Unclean One, you briefly enter the realm of Nurgle when you jump into the ritual circle…so it’d add so much to the atmosphere if some Nurglings were present as well.