My half-baked Stranger Things Vol 2 Predictions

Henry has been possessed by the Mind Flayer the whole time, akin to Billy, Will, etc. He was initially infected when he met the Mind Flayer in Dimension X. The Mind Flayer plans to merge Dimension X and the real world so that it can take over the Earth, using Henry to do its bidding in the human world - Henry is basically the psychic bridge between the Mind Flayer and its victims. Its goal this season is to build a wormhole that connects Dimension X and the human realm, and it needs 12 kids to power the opening of the gate - similar to how Vecna created the gates by inflicting psychic damage on his victims telepathically, the Mind Flayer wants to accomplish this on a larger scale - by plugging the 12 children into the hive mind physically through the spire vines, and then having Henry form a psychic connection between them, this can open a gate from Dimension X to the human world (not sure exactly how this works). Since the wormholes gravitational force would tear itself apart the instant it formed (as Mr. Clarke said), I think it’s necessary that the Mind Flayer gives the kids psychic powers to somehow prevent this from happening? Not all of Henry is under the Mind Flayer’s control - I think he actually is acting to save Holly and the other kids by holding their conscience in his mind while they’re physically trapped in the upside down and connected to the spires. Henry also gave Holly some sort of music player, which I think signals that he intends to help her escape the Mind Flayer. 

I think Will was originally taken to the upside down to construct the wormhole, but Henry allowed him to escape somehow. I think Henry chose Will because he reminded him of his uncorrupted self, before he was possessed by the Mind Flayer - Henry knew that choosing Will to open the wormhole by plugging him into the upside down physically would result in him gaining access to the Mind Flayer’s power (and Henry's own psychic abilities), and he believes that Will is special enough that he will eventually be able to wield his own psychic power to help him defeat the Mind Flayer and free Henry from its grasp. So maybe when Vecna says that he and Will are going to do beautiful things together, he is speaking as both the Mind Flayer and Henry - the Mind Flayer believes that he’s the first step towards the wormhole to bridge Dimension X and take over the human world, and Henry believes that since Will now has access to his psychic powers through the hive mind, he will eventually be strong enough to free him from the Mind Flayer.

I think that Holly, Max, Will, Eleven, 008, and Henry will battle the Mind Flayer. Holly, like Will, will be able to harness Henry’s power too - I think that’s part of why he’s so close to her. Plus, Mike had a bit of a foreshadowing moment with her, calling her a cleric who can channel divine power. I think Holly will help free Henry from the Mind Flayer’s grasp in some sort of emotional moment.

Max and Holly make a plan to escape Henry’s mind - maybe they try leading Henry to the cave somehow and getting the memory of the Mind Flayer possessing him to play out, which causes the Mind Flayer to see that Max is in Henry’s mind and alive - he sends the demodog to kill her at the hospital. The Mind Flayer sees that Henry is sheltering Holly in his mind to protect her, betraying him, and the three of them run through Henry’s mind, trying to escape the Mind Flayer. I think Lucas will know to play Kate Bush for Max when the demodogs attack (he’ll know she’s probably being attacked psychically as well), and I think she’ll wake up from her coma while he’s dramatically fleeing a demo attack. But Holly won’t hear the music since her body is in the upside down, so she’ll be stuck fleeing the Mind Flayer with Henry in his memories.

Simultaneously, El, Hopper, 008, Nancy, Dustin, Steve and Jonathan meet up in the upside down - they’re still trying to figure out how to get to Holly. Maybe 008 creates illusion gates or some sort of illusory upside down rift disaster to divert the military presence. Or, maybe they escape the upside down through a gate left by some demogorgons or demodogs, especially if there are a lot of them going after Max.

I think El will piggyback off of Will’s connection to the hive mind to find Holly. While she’s in there, the Mind Flayer senses her presence and sees what she’s doing. This is when Will is possessed by the Mind Flayer and screams “RUUUN!” because he senses the Mind Flayer taking over him. Maybe he has an epic fight scene against El - Mike watches in horror as the two people he loves most face off.