We probably have at least a year and a half, if not two years until the season 5 premiere of the show, based on how the seasonal releases have gone to date, and comments from the cast.
There are some recurring themes that seem to be based on things the Duffer Brothers have said, partially based on in-show clues.
There is a growing theory that I like a lot about season 5. Go through the pieces.
The idea is that these two things are related.
One enduring mystery about the Upside Down that was brought up again this season was that it is a flipped version of Hawkins at a point in time. Nancy's sophomore year is when the show began.
The Upside Down was not created by Eleven, but by opening a portal to it, which allowed Henry/Vecna to exist there.
The Upside Down is a projection of a real-life D+D campaign of which Will is the ultimate dungeon master, even if he doesn't realize it. The moment when he was most happy was when he was playing D+D with his friends. He froze it in time when he was there.
One idea is that if the gang returns to the Upside Down, Will may discover that he has a lot of power there, something that only hints at. It's possible that Eleven is the key to the upside down. It is possible that he was involved in the creation or existence of it as a result of a D+D campaign.
I don't know why Will has this power. The origins of Will are not a mystery. He has two siblings. They don't appear to have any special powers. His father wasn't a magic being. There are gaps Will is too much of a focal point for the Upside Down, someone that Vecna is not only connected to, but may in fact fear a bit. Fans have noticed that the red dragon Will drew in his artwork is the only other one in the D+D campaign that is more powerful than Vecna.
The main Will storyline this season was that he appears to be on the verge of admitting to having feelings for Mike, which does not seem like it will work out, given Mike. His brother seems to more or less know what is going on. I wonder if the idea of love breaking through the dimensions will affect Will's feelings in season 5.
All the pieces are difficult to put together, but there is something here. I have nothing else to say about keeping an eye on Will.
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The Herokiller series and The Earthborn trilogy are available for purchase.