1. Eleven's powers are tied to the Mind Flayer

It is quite the coincidence that El's powers go away when the Mind Flayer is "defeated" and the bridge to the Upside Down is closed.

Eleven has had probably the most contact with the Upside Down and the monsters that reside there, so I wouldn't be surprised if her abilities were tied to it too.

This means that in season four, if they were to close the bridge for good, she would become a normal girl.

2. Dr. Brenner is a Russian prisoner

During the after-credits scene of the season finale, we see a Russian base with guards mentioning an American prisoner.

Though many think this American is Chief Hopper, many also think it's Dr. Brenner - Eleven's "Papa" and the former director of the Hawkins lab.

Though it seemed like the Demogorgon killed him in season one, it has been confirmed by the writers that he is in fact still alive, making him the more likely candidate.

3. Hopper is alive and in the Upside Down

In "Stranger Things," it's basically a rule that if you don't see a body, they're probably not dead.

Though it seems like Hopper was done for when Joyce closed the door to the Upside Down, there's a lack of a body. Also, the camera lingers a bit on the crack in the wall closing up after the dramatic door-closing scene, implying that Hopper could have slipped through the door while it was happening.

This would mean that Hopper's alive (though definitely not safe) in the Upside Down.

4. Season four will deal with the Chernobyl disaster

The nuclear energy disaster definitely fits the timeline of the show, having happened in 1986, while the third season of "Stranger Things" took place in 1985.

Some think that the Russians will open another door to the Upside Down at Chernobyl, saying that it was a nuclear accident to cover up what really happened.

Also, with the introduction of Soviet agents and scientists this season, they will definitely need to address it in the show.