Well, many fans think that the season ended with the death of one of our favorite characters but this can also be a false assumption. Jim Hopper was one of the most loved characters in the show.
He has been the prime person to investigate all the happening in Hawkins making him a very important part of the series but in the last episode, we see that he is in a fight with a Russian spy. They take the fist too far being too close to the machine that was made to open the gates of the parallel universe, Joyce, however, is seen in the control room trying to put an end to the machine to stop and seal the wall once again.
With no time left for Hopper to return back to the control room, he tells Joyce to let him die and stop the machine as they were lacking time and the Demogorgon was getting stronger and attacking El, so she does it and we see the machine blasted which caused the death of Hopper.
Now when we all saw the post-credit scene which showed us a Russian prison and the guards were going to take a prisoner out and the other one says not the American take the Russian one out and they take him to an abandoned cell and feed him to a young Demogorgon.
Well seeing this fans speculated that the American prisoner can be Hopper and gained hopes that we will see Hopper returning in season 4 as well.