On Nov 6th, 1987 Vecna will bait Will to power a wormhole that will enable Vecna to time travel to ---?
[Erica] "Wormholes are neat because they allow matter to travel between galaxies or dimensions without crossing the space between." - [Mr. Clarke] "Yes. Exactly right. Again. And, uh, just think of all the places mankind could go. Another galaxy. Another time, even."
Mr. Clarke's classes have in past seasons always been used to foreshadow events that would happen later in the series, like in S2 when he's teaching the party about viruses and how they respond to fear, which is then applied to a possessed Will and his aversion to heat. This year Mr- Clarke is teaching his class about Wormholes, and specifically mentions them enabling time travel.
On the board, we see that there will be a test on this topic on Monday, November 6th.
Holly Wheeler is kidnapped on the evening of November 3rd. The following night (November 4th), Vecna attempts to kidnap Derek and succeeds in kidnapping 3 other children. On the evening of November 5th, he kidnaps the remaining 7 and Derek, and Will awakens his powers.
12 children on spires in the shape of a wormhole, 12 numbers on a clock. Wormholes can enable travel between dimensions and time.
It's important to note that there appears to be a 13th spire in the middle of the cave that Vecna has imprisoned the children in, which Will is standing right in front of in the picture above. I believe that this is foreshadowing that either Will or Vecna will be attached to that spire at some point. If it is Vecna, then the purpose of that spire is likely to be the actual wormhole and transport Vecna to another time, if Will is attached to the spire, it's likely as a power source to fuel the wormhole. The reason it's unclear whether it's Will or Vecna that is the one that will be attached to the spire is because of Mike's comment about how Will is like Vecna:
This means that when Will is standing in front of that painting of the cave, he could be used as a sort of substitute for Vecna.
Regardless, it's clear that Vecna's goal is to travel to another time, and the fact that it gets activated on November 6th seems to be important, since that is the day Vecna is building this up to and the day Will will likely be attached to the wormhole as well. However, I don't think Will will be kidnapped again like the other children. I think it's much more likely that Vecna has already started luring Will there, and Will will go in an attempt to rescue Holly and the other children, unaware that this was Vecna's plan all along.
Vecna is clearly aware of Will's ability to Spy on him and the Demogorgons. He also seems to be aware of Will's 'potential' abilities going by how Vecna tells Will that Will showed him what he could achieve, and this doesn't make much sense if it only refers to Vecna's ability to spy through those he curses because Will is not the first person he has ever attempted to kill through his curse. As a matter of fact, he's never tried to kill Will through the curse, so Will wouldn't bring him any insight on how that works.
In season 2, the MF has Will tell Owens and the soldiers that there is an important place in the tunnels that the MF didn't want him to see. Because of this, many of the soldiers head there only to discover that this was a trap and then get eaten by Demodogs. The same thing is being set up for Will and the wormhole cave.
These two sets of dialog are so eerily similar to each other, it's not even funny:
S2: [Will] "That's it." [Dr. Owens] "That's what? What... what's there, Will?" [Will] "I don't know. I just know he doesn't want me to see there. I think it's important." -------------------------------------------------------------------------- S5: [Joyce:] "What is it?" [Will] "I don't know, but I could feel it's important to him. [Derek] "To Mr... Whatsit?" [Will] "His real name is Henry. [Robin] "AKA Vecna." [Erica] "AKA One." [Derek] "That's too many names." [Erica] "Why won't you just zip it and listen, dipshit?" [Will] "I had this feeling he didn't want me there. I wasn't supposed to see it."
Except in this case, Will will be going to the wormhole-cave to try and rescue Holly and the other children, only to discover that it was a trap that Vecna lured him into so that he could use Will as a power source for his wormhole.
There is also the fact that Will seems to be just fine out in the cold again:
Just like when he was a spy for the Mind Flayer.
Note: I don't think Will is actually possessed by Vecna like he was in s2 by the MF, as this wouldn't explain why the Demogorgon only knows where the shed is once Derek is awake. However, Will seems to be able to feel when Vecna activates his powers, and this seems to also trigger his powers activating and enables him to look through the eyes of the Demogorgons. In the same way, I think Vecna would be able to feel Will using his powers (whether he's conscious of it or not), which is how he knows about them in the first place.
And I believe Vecna will succeed in creating the wormhole, which would explain this scene from the Official Trailer for volume 1:
And this follow up scene from the Volume 2 Trailer where you can see the UD actively melting:
This isn't something that's been happening for a long time, we can see in these two clips (which are chronologically directly after each other, you can clearly see this in the vol 2 trailer) that the UD has only just started melting, and rapidly at that.
This might also lead into these scenes, although the second one conveniently cuts off right before we can see what happens to the UD wall:
I think this could very well be the UD turning into the wormhole, allowing Vecna to travel to another time. However, even if Vecna initially succeeds with creating the wormhole, I don't believe he will actually be able to use it, or is at least is interrupted while using it.
The Einstein–Rosen bridge ('Wormhole') was discovered by Ludwig Flamm in 1916, a few months after Schwarzschild published his solution, and was rediscovered by Albert Einstein and his colleague Nathan Rosen, who published their result in 1935. In 1962, John Archibald Wheeler and Robert W. Fuller published a paper showing that this type of wormhole is unstable if it connects two parts of the same universe, and that it will pinch off too quickly for light (or any particle moving slower than light) that falls in from one exterior region to make it to the other exterior region. (wikipedia)
[Mr. Clarke] "Einstein-Rosen bridges, more commonly known as wormholes, [...]"
It's interesting that Mr. Clarke makes specific mention of talking about Eintein-Rosen bridges, which have been shown to be unstable when connecting two parts of the same universe by John Archibald Wheeler. This hints at one of the Wheelers being the reason for destabilising the wormhole, and preventing Vecna from completing his plans. Karen and Ted are obviously out of the picture, and Nancy seems very unlikely, therefor I think this will either be Mike by appealing to Will then Will being able to take control over the wormhole, since he has the same powers as Vecna and should therefor theoretically be able to do so, or Holly by somehow connecting both ends of the wormhole to the same dimension (or time) as this is the power Mike describes Holly the Heroic to have in ep 1.
I think it's much more likely for this to be Mike, since we have already gotten hints at Holly using her power, but only in memories, i.e. her imagination.
We can see that she's with Max in this scene in her outfit that she has been wearing in the Creel house in Vecna's memories, so I think it's unlikely that she actually has any powers in the real world that could do this, she only has them when she's in a memory and can use her imagination to go to another memory.
However, what I could see happening is the following:
There might be a reason Vecna needs Will, someone with his same powers from the MF, to open and power the wormhole in the first place. Theoretically, if Will and Vecna's powers are identical, Vecna should have no use for him. But, Einstein-Rosen bridges specifically destabilise when connected to the same dimension at both ends. We know that Henry doesn't actually want to go to another dimension, he wants to go back in time. In this case, we would need to assume that they changed the way wormholes work in the show from how they would theoretically work in the real world, in the sense that instead of destabilising when connected to the same dimension twice, they destabalise when connecting to the memories of the same person twice. What I mean by that is: If Vecna were to be the one on the 13th spire, he would be the one powering the wormhole and therefor it would be connected at the starting end to Vecna's memories, and the destination would also be his memories, therefor the wormhole would destabilise. So Vecna needs Will, someone who now has his same powers, to be the starting point so that the wormhole is not connected to the memories of the same person twice, and remains stable. If that is the case, I could see Holly somehow accidentally affecting this while she and Max try to escape Vecna's mind, and that would end up destabalising the wormhole.
But I think it would be much more fun and logical to have Mike reach out to Will while Will is (involuntarily) powering the Wormhole and be able to bring him back to his senses, much like when Will is possessed by the MF in season 2. Will could then change the destination of the wormhole, or stop it from fully forming in the first place.
In short: Vecna kidnaps 12 children during the nights of November 3rd, 4th and 5th, in order to open a wormhole on November 6th. To power this wormhole he is luring Will into the UD and to the wormhole-cave by deceiving him. Vecna shows Will the cave and lets Will feel as though this place is of importance and that Vecna did not want him to have this information. However, Mike will be able to stop Vecna's plan from succeeding by appealing to Will, who is then able to take control over the wormhole, and stop it from going...where, exactly?
Honestly, this part remains a complete mystery to me. There are several key dates within the show (and the play) that Vecna might want to travel back to, however I cannot settle on which I find the most plausible.
1) November 6th, 1959 / the day Henry killed his sister and parents We have several conflicting sources on when Vecna actually kills his parents. In season 4 ep 3 we see 2 news paper articles discussing the murders, one from (presumably) Thursday, March 14th 1959 and one from March 26th 1959. The article from March 14th dates the murders to the previous Saturday, March 11th 1959.
However, according to Stranger Things Wiki, TFS dates the Creels moving into Hawkins to October 1959. Season 5 seems to support this, we can see that Joyce's play (which Henry acts in) will be performed on November 6th, 1959, and I doubt she would already be distributing Flyers for the play in early March
Obviously November 6th is also the date El opens the portal to the UD and Will goes missing in 1983.
2) March 22nd, 1979 / Will's 8th Birthday as mentioned by @greenfiend in this post, 1979 seems to be a year in which a lot of things change and are forgotten about. Most notably it is the year in which every member of the party turns 8, which is especially significant for Will. In s5 Joyce says that Will was 11 when he went missing in 1983, even though he was 12 at the time.
Obviously, Will's birthday also seems to be forgotten about by everyone, including Will, in season 4.
-> March 22nd, either 1979 or 1986 could also be the memory Will ends up going to instead if he takes control of the wormhole and changes its destination.
3) September 8th, 1979 / the day El first banishes Henry to Dimension X The timeline from the Stranger Things Wiki dates Henry's banishment to September 8th, though I could not find the precise moment this is shown in the show myself. I did find a previous recording that Dr. Brenner has El watch that is dated to September 5th, so this seems plausible.
4) The day Henry Creel is possessed by the Mind Flayer, 1953 (Stranger Things Wiki) As far as I'm aware, we do not know the exact date when Henry is first transported to Dimension X and then possessed by the Mind Flayer. If it's the same date as any of the other dates I have mentioned so far, that would obviously be another reason for Vecna to travel back to that date.
5) The Future Since we're talking about time travel here, the future is obviously a possibility, but I think it's very unlikely because it would be very out of the blue and there isn't really any future event that's been mentioned that could be important enough for Vecna to travel to.
...But why?
What is Vecna's reason for wanting to time-travel in the first place?
I think, while depending on what date Vecna will end up trying to travel to will certainly be related to his reasoning, no matter what, one reason will be getting out of the control of the MF
[Dustin] "How did Watergate open up? Only two gates have opened, so far as we know. One by El, one by the commies. But it's not the commies or El this time, so it... Holy shit. Wait, wait, wait, wait." [Lucas] "Wait, wait, wait, what?" [Dustin] "There's one thing we've never understood. Which is why Vecna's killing people. What's his motive? Killing teens? It always just seemed too random. Too prosaic. On top of that, how does the Mind Flayer figure into all this? Maybe this is it. This is the answer." [Lucas] "WHAT is the answer? [Dustin] "Okay, just hear me out. How did El open the Mothergate?" [Lucas] "She contacted the Demogorgon." [Dustin] "With psychic contact. Just like..." [Lucas] "Vecna when he casts his spells." [Dustin] "Exactly. So, what if, with each kill, he's not simply killing them, he's making a powerful psychic connection with his victims? A connection powerful enough to rip a hole in the fabric of time and space." [Lucas] "He's opening more gates." [Dustin] "Bingo. Why would he be opening gates?" [Lucas] "To take over the world." [Dustin] "Who do we know that wants to take over the world?" [Lucas] "The Mind Flayer." [Dustin] "So if the Demogorgon was just his foot soldier, Vecna's his five-star general. A five-star general with the power to open gates." [Lucas & Dustin] "Holy shit."
Aside from this, TFS also confirms that the Mind Flayer is the one actually pulling the strings, not Vecna. At this moment, we don't know how much consciousness/control Vecna actually has over what he's doing, but if he has any at all, I think it's very likely that, regardless of how evil he actually is un-possessed, he doesn't like being under the control of the Mind Flayer and wants to get out.
Scenes from vol 1 and the trailers that could also be related to the wormhole:
1)
Derek looking at a Tesseract-like structure. This structure represents the fourth dimension, which is the expansion of our 3 dimensional world by time. Dimension 1: length (e.g. a line), Dimension 2: width (e.g. a square), Dimension 3: hight (e.g. dice), and Dimension 4: time
(wikipedia)
2)
From the Teaser, potentially a scene where Will is being trapped/already trapped by Vecna and being used as a power source for the Wormhole, In combination with these lines from Vecna which were used in the Official Trailer out of context:
[Vecna] "William, you are going to help me... One last time."
3)
[Dustin] "This whole time, everything we've ever assumed about the Upside Down has been dead wrong."
Possibly referring to how, until now, we've (and the Party) assumed that the Upside Down is it's own dimension that is frozen in time, but it is actually instead a connecting dimension between Dimension X and the Rightside Up, that can function as a wormhole to enable time travel.
4)
This scene from the Official Trailer of a person falling somewhere through something, potentially Will or Vecna using the Wormhole, though I think it's more likely to be Will since I don't think Vecna will actually fully succeed with his plan. They also seem to be flailing and I cannot imagine Vecna doing that.
5)
These scenes from the Volume 2 Trailer, in which the Upside Down seems to be getting destroyed.
Particularly that 3rd one is interesting, because it seems to be what happens before the person falls through the sky (?), and might be the wormhole opening.