Spoilers ahead for Stranger Things 3. Good news, Hopper fans: the Stranger Things 3 end credits scene could mean your fave is still alive. However, in order to fully understand it, we need to revisit how the first two seasons have ended.
In Season 1, the final moments of the show revealed that Will could still see glimpses of the Upside Down, even though he was back in the real Hawkins. His family sat cheerfully having dinner together, but darkness was lurking inside of him. That extended into Season 2 when Will eventually became possessed by the Mind Flayer. Then, at the end of Season 2, the final shot showed the Mind Flayer looming over the Upside Down-version of the town. It was a signal that the kids weren't free of the monster yet. Season 3 built on that by having a Mind Flayer for them to deal with, even if it wasn't the Mind Flayer from the Upside Down. The actual shot of the monster and the school didn't appear in Season 3, which hints that perhaps final scenes are more of a suggestion of what's to come than a literal snapshot of the future.
The end credits scene in Stranger Things 3 is similarly ominous. After the credits roll, a title card comes up reading "Kamchatka, Russia." We see snow billowed up over bunkers, insinuating that we're being shown some sort of army base. Inside a bigger building, rows of steel-doored cells line the hallway. Two guards come to choose a prisoner to take away. "No, not the American," one guard says, instead opening the cell next to the one holding the mystery American. A prisoner is dragged away, begging for his life and claiming to be innocent. The guards don't listen and put him in a cage, then release a Demogorgon from a trap door inside.
A couple of things here: firstly, obviously the Russian part of this story is far from over. The Russians may not have succeeded in opening the Upside Down gate in Hawkins, but they already have a Demogorgon back in Russia. El and her friends think they closed the gate and finally got rid of all the monsters, but some are clearly still lurking in the real world. Secondly, the "American" in the cell could very well be Hopper. His friends may believe he's dead after the machine explosion in Episode 8, but we never saw a body. He could have easily been taken hostage by one of the Russians in the mall and shipped off to Kamchatka to await his trial by Demogorgon, or to be held captive for something even more nefarious.
However, just like how the Mind Flayer scene with the school didn't come to exact fruition in Season 3, don't expect Season 4 to rush to explain this Russian base camp. The mystery of the Upside Down and its creatures goes far past Hawkins, and it likely can't all be explained easily in Season 4's first episode. But hopefully, if it is Hopper that the Russians have, he can figure out some kind of escape soon. He's dealt with enough monsters already. He doesn't need another Demogorgon run-in now. And after losing her powers and having to move away from her friends, Eleven needs her dad more than ever.