Okay, serious fan theory question for those who watch Stranger Things, and for those that do not, please ignore this for it contains spoilers for the end of season three.
At the end of season three, Jim Hopper is seen in the room with the particle machine keeping the gate to the Upside Down open and some Russian soldiers in that room, but what we DON'T see, is Hopper dying, it's just assumed with piles of ash. Before the end credits are finished, we are taken to a Russian secret base, with two officers walking past cells. One of them goes to open a cell, but the other stops him, telling him to leave the 'American' alone, before they ended up grabbing a Russian prisoner, and feeding him to a Demigorgon. I don't know about you, but I think the 'American' they were talking about was Jim Hopper. I think they managed to somehow get Hopper out of there, before the machine self destructed, and plan on using him. I may be wrong, I don't know. Please, if you have any other theories, please let me know.