A pretty prominent and loud element of the teaser is the presence of a grandfather clock buried within a tree. Throughout the short 45-second video, the ticking of said clock can be heard in the background as its hands tick past 11 o'clock. With every tick, the surrounding forest seems to fall into decay, with the familiar vines of the Mind Flayer wrapping around everything — even the show's logo. Soon the entire scene is corroded, and then the screen goes black, and the bell of the clock tolls, signaling the new hour.
This could be a reference to a few things: a metaphor for Eleven leaving behind her persona after she's lost her powers, the introduction of a new powered individual like Eleven (Number 12), general time passing by, or more interestingly, a time jump!
We already know that each season follows a seasonal timeline. But with the circumstances of Joyce moving the kids to get away from Hawkins and dealing with a rapidly growing cast, it wouldn't be farfetched for them to go beyond the standard year time skip. Imagine coming back to Hawkins in a time when the remaining members of the Party (Mike, Lucas, Dustin, and Max) are older and have become complacent in the absence of the Upside Down. They wouldn't be able to sense the danger coming their way until it's too late. That would really take the It references to a new level!
The ticking clock could also be a signal that someone's time is running out. The most obvious suspect is Hopper, who is undoubtably in trouble as everyone tries to figure out where he is and rescue him. It could also be that the town of Hawkins itself is running out of time, as it's once again taken over by the darkness of the Upside Down and all its creatures. It definitely seems like an old foe will be making a comeback that spells trouble for the sleepy little Hellmouth town.