If the sound of the clock isn't deafening enough, the teaser also features the growling of a demogorgon. We thought we left the terrifying creatures back in season one, but as the season three postcredits scene revealed, they're definitely still around.
Obviously, the Russians aren't done with Hawkins or the Upside Down just yet, and we saw that they have a demogorgon in their prison. We can't put anything past them, so it's highly likely that they're planning on getting/creating a whole lot more. There's no reason why they couldn't create a whole army of demogorgons, especially if they have a certain doctor working for them as well. And who knows what evil things they could do with even more demogorons at their service after they open another portal?