Barb in Season 2
It was announced that Barb will not be returning in Season 2 of Stranger Things. But I think she will.
See, if you remember, when Will was in the Upside Down, the Demogorgon had a tube implanted into him feeding him slugs. It had an Alien feel to it, where will was a breeding body I think. Barb was in more of an intense state compared to Will, but those slugs must hatch into actual creatures. At the end of Season 1 we saw Will vomiting up those slugs, which I think was a way of both his body rejecting them and the slugs rejecting him ready to hatch BECAUSE he was alive. So with Barb being confirmed dead, they will hatch inside of her, and technically, hatching AS her.
So yes, BARB herself will not be returning in Season 2. But I think there is a possibility that she will be the Thesylhydra.
Because think:
Before Eleven, Brenner had a whole crew of people trying to make contact with the Upside Down, and they all “died” in there. So what if they all got the slugs inside of them and was a feeding ground for the slugs. But only one actually turned into the creature we know as the Demogorgon.
Think of the slugs as a symbiote almost, like Venom in Spider-Man. But said symbiote can turn the physical body it is hatching within into a creature, and each creature being different.
It’s not unconfirmed and I think it’s plausible.