This is my full theory on the events of ST4. It's nuts.
So, the machine brainwashed Hopper is helping build is a machine to harness the power of the Mind Flayer, which the Russians will use to possess El and kill Jonathan. The American turns out to be Brenner, who taught them how the gate works and crap (see theories by @Jtell23, anything that's part of this theory that I forgot to put in is in one of his posts), but the Russians are probably gonna betray him. He'll then help Hopper get his memories back, they'll have a heart to heart talk or something maybe which will reveal Hopper's backstory as said by the Duffers. They'll team up to hijack the Russian machine, which Hopper will use to possess Will and tell everyone where he is and how to save him. Joyce and Murray will head to save Hopper, which they will be successful in doing.
I believe that Brenner will die. He could be captured by the Russians and fed to the Demogorgon for betraying them by helping Hopper escape, or he may manage to get back to America, where he would most likely be killed by El (whether or not she is possessed at that time, because she obviously hates him).
When it comes to what is happening in America with the party, obviously Mike and Dustin will be in the Hellfire Club, the Hawkins High DnD club, while Lucas will most likely be on the basket ball team (Tiger Pride!). Mike, Dustin, and Erica, as well as the entirety of the Hellfire Club will be accused of Satanism, as DnD players were in the eighties. Having a demon as their mascot and the word "hellfire" in their name certainly won't help. Not sure what's going on with Max, other than she's still with Lucas probably, but I don't think much will be new with her. Dustin is still with Suzie, and with El being possessed by Russians and out to kill, who knows where Mike's social life will be. Will I feel like deserves to finally get a girl, he deserves it if he's gonna be possessed another time, so maybe that girl who cried at his funeral who I forget the name of will be with him, all though if Will is living in either Maine (where Bob wanted to move) or California (mentioned in a previous leak on this page), a relationship that long distance may not work. Oh well, poor Will, girfriendless and constantly loosing control over his physical body.
Also, y'know, with Will getting possessed again (at least this time it's Hopper and not a monster) and Jonathan being brutally murdered (how do y'all think El will kill him?), Joyce will be even more mentally unstable than before, and she's gonna go all Rambo on those Russians.
So, I think that's everything, right? I'll leave the way this crazy story ends up to the comments, and the show itself. I hope you enjoyed this weird collection of fan theories and leaks that form a loose story!