Many aeons ago, the creative minds behind the show teased the inevitable return of the smoke monster aka the Mind Flayer. "They've shut the door on the Mind Flayer, but not only is it still there in the Upside Down, it's very much aware of the kids, and particularly Eleven," said co-creator Ross Duffer. "It had not encountered her and her powers until that final episode. Now, it knows that she's out there." It seems this freaky monster is determined to rip a new hole in the fabric of our universe, if only to get to Eleven. And since Will's tiny body housed the creature for a period of time, he must be sensitive to it as well.
Is there a chance the Mind Flayer could cross over into this world? Well, the Demogorgon made it to our side, so why not? As an added wink in the poster for season three, Eleven and Will seem to be the only ones aware at all of the evil lurking in the photo. While everyone else stares out into the distance, these two stare back at whatever lurks beyond the borders of the poster.