Curently Nurgle affiliated chaos mutants use flamethrowers as their suppresion weapon.
why not turn that into a more lore friendly and nurgle friendly bile thrower? Instead of flames, vomiting a mixture of acids, bile and diseases filled soups cooked up by the plaguefather? While on the topic: nurgle havocs and plague marines in general have acces to bile-throwers as their patron unique weapons (much like slaneesh gets sonic weapons, tzeench sorcery and khorner psyker resistant collars and chain-axes). In game they have plague grenades and ... what do nurgle havocs actualy shoot? Outside of the grenade spray they do from time to time. They look like they are holding a heavy bolter but I haven't seen them fire it like one. They could be given bile launchers and/or if using rocket launchers (I suspect they actualy shoot rockets) then plague based munitions?
Just theme discusions here. Flames generaly are associated with cleansing disease and taint, yet here we have nurgle units using fire when would it not make more sense for them to spread disease instead of fire?