If they added a Plague Doctor mask to the actual game, they’d also have to add a new type of enemy that wears it. Such a mask doesn’t really fit to any existing enemies.
But it was suggested multiple times to add a plague doctor mask to a possible third Loyalist Pack. I support that idea. Illusion ideas for future Loyalist Packs
edit: I guess they could add the mask to a faction vendor instead of an enemy, but I still like the loyalist pack idea better