Just as Hopper and Joyce are about to simultaneously turn the keys that will kill the machine that Russian spies have been using to let the body-snatching mind flayers into small town Indiana, the Terminator-like lackey who has been tracking our protagonists appears. He and Hopper do battle, putting them on the platform right next to what is set to go kablooey. By the time Hopper kills his foe, it's too late for him to reach safety, Joyce must switch the device off. When she does the room is obliterated, and when she looks back for Hopper, he's gone.
The conclusion of the season finds the Byers, now with El as part of their family, in mourning as they prepare to finally get the hell out of Hawkins. As they pack, Joyce finds a note from Hopper, the emotional speech he was going to make to El. It's a touching coda to his story, and yet something doesn't feel as final as maybe it should be.
Cue the mid-credits sequence. In a Russian facility of sorts a guard calls for a prisoner. "Not the American," he says, and, lo, a random poor soul is fed to a Demogorgon. The surprise of the sequence is the reappearance of the season one bad guy, but the "American" comment is more of a clue as to what's to come. Who is "the American?"
The most likely answer? Hopper. Because, come to think of it, we never actually saw him turn to goo, the way some soldiers did. Maybe he got sucked into the Upside Down when the gate closed and the Russians fished him out of it? Or maybe someone grabbed him in the process of escape? Compare Hopper's death to that of Billy (Dacre Montgomery), who gets impaled in multiple ways by the Mind Flayer: One is pretty clearcut, the other is, well, not.
Harbour could very well return, especially given that Stranger Things often takes its time between seasons. A fourth season has been teased and is essentially a given even, though it hasn't been officially announced by Netflix. Harbour's currently in the middle of filming Marvel's Black Widow movie, but otherwise, his slate is free. (Alas, it seems like a Hellboy sequel is unlikely.)
Still, there's something a little disappointing about Hopper's likely return. It undercuts the effective moments of sorrow that made up the end of the season. How are characters supposed to grow if their tragedy is just one big fake-out to appeal to fans?