Stranger Things Season 2: 9 Crazy Fan Theories (That Might Actually Be True)
6. Will Is A Vessel For Monsters
After his stint in the Upside Down, Will returned to this world, but clearly hadn't fully moved on from his experiences. This is likely to play out on a psychological level, but also on a more physical level as well.
We saw him at the end of Season 1 coughing up a slug-like creature, which has led to various types of speculation. These range from it simply being an after-effect of his time in the Upside Down, all the way to Will actually being a monster himself now, but the best idea lies somewhere in the middle.
What if Will isn't a monster himself, but is actually a carrier? Given the amount of time he spent in the Upside Down it makes sense something could've happened to him on a more biological level, while we also see an insect-type creature on his mouth when he's found. With the lines set to blur between the real world and the Upside Down, he could be responsible for bringing more creatures than just the Demogorgon (or perhaps baby versions of that) into this world.
This is expanded upon here, by Redditor angelfacedweirdo, who posits that the Demogorgon itself uses its victims as reproductive opportunities, which would then lead to new Demogorgons in Season 2.