If you don't remember, Dustin takes a spray to the face from one of the Upside Down tentacles in the season two finale, and I can't imagine that won't come back into play. He's not the only one, either; Hopper gets a dose earlier on and it renders him unconscious for an undetermined amount of time! What if they fall prey to the same kind of possession Will experiences in season two? What if it's something even more sinister than that?! I could see this kind of thing spreading throughout the town, forcing the government to quarantine the population. Wouldn't that be a great set-up for season four?