That Stranger Things 3 postcredits scene, explained

The credits may have rolled, but thanks to an extra scene, speculation over the next season carries on.

As a heavyhearted Joyce Byers closes the door on her empty house in Hawkins, Indiana, Stranger Things 3 comes to an end. The triumphant We Can Be Heroes fades behind the whistles of an icy wind as the end credits begin to roll -- wait, hold up. In true Stranger Things fashion, there's an ominous mid-credits scene to interrupt proceedings.
That's right, despite the heroics of Eleven and the gang, danger and Demogorgons are never far away. Let's run through what happens in that mid-credits scene, and what it might entail for our lovable, bike-riding misfits.

What happens
In the dark, icy plains of "Kamchatka, Russia," we see what appear to be the domed buildings of a military facility. In one of the them, two Russian soldiers stop to unlock a steel door. "No. Not the American," the second soldier says, no doubt a massive hint our favorite police chief is still alive and lies within, but more on that below.
The soldiers instead move on to the next door, clanging it open to reveal a haggard prisoner who begs in Russian not to be taken. Swiftly ignoring him, they drag him downstairs into a cage, where one of the soldiers cranks a wheel that opens up a metal hatch. Out prowls what appears at first to be a brawny, goo-slobbering Demo-Dog, until it proceeds to stand up on its hind legs as a fully grown Demogorgon. It opens its four Venus flytrap flaps for a head and the prisoner is toast.

What it means
More Demogorgons
First and foremost, it's pretty clear there are more Demogorgons knocking about. While El defeated the Demogorgon terrorizing Hawkins in season 1 and sealed the town's gate between dimensions in 2 and 3, it's honestly not surprising more monsters have crept through from the Upside Down.
The Demogorgon in Russia suggests another wound between dimensions has been opened. That would allow the Mind Flayer, controller of the Demogorgon hive mind from its home in the Upside Down, to continue its plans for world domination. This is assuming there's only one Mind Flayer. This is also assuming the Russians haven't figured out how to use the Demogorgons for their own means, akin to Dustin befriending our favorite loyal Demo-Dog Dart. It is the Cold War, after all.

More Russian military
The Russian military may yet be done with trying to take advantage of the gate to the Upside Down under Hawkins. We now know they've been experimenting in Kamchatka, which in the real world is a remote, volcano-filled peninsula in Russia's Far East. During the Cold War, it was developed as a secret military region that had prime positioning across from the US.
Could a mission to a military base on lockdown overseas be in the cards for season 4? It's difficult to imagine El and the gang, as smart and powerful as they are, biking their way through that one. A more likely play seems to be further attempts from the Russian military at exploiting Hawkins' interdimensional gate.

Hopper's still alive
At the end of season 3, Hawkins' resident unsmiling police chief plays a crucial part in stopping the Russians from tearing open the wound to the Upside Down. While Hopper struggles with the relentless Terminator-esque Russian officer down by the energy drill, Joyce is up in the control room cutting off the power. In the ensuing energy surge all the hazmat scientists explode in blood and guts. Joyce stares at the spot Hopper had been standing in, but he's nowhere to be seen.
Crucially, there's also no sign of Hopper's blood and guts, so that leaves his possible death open. Again, the line from the Russian soldier -- "No. Not the American" -- heavily suggests he's been captured, probably while Russian soldiers cleaned up their botched mysterious doings. As for speculating about Hopper's future, a healthy dosage of torture and manipulation is probably in the cards.
There's also the matter of David Harbour's future projects, which probably won't see him tied to a huge Hollywood franchise with Hellboy. The determinedly gory R-rated film bombed with critics and the box office, despite Harbour shining as the red devil.
Harbour seemed to confirm the franchise's fate when he weighed in on the possibility of a sequel in June. "I don't think the perception was that it was a hit, and so in that way, I don't know that the risk is worth it," he said.
Will we see Hopper again? Whatever the case, creators the Duffer Brothers have suggested Stranger Things will wrap up the stories of our quickly growing young heroes after a fourth or fifth season.