In Season 4 of Stranger Things, fans were shocked after learning the origins of Vecna. It was confirmed that he was Henry Creel, who later became One. After committing mass genocide at the Hawkins Lab, he was banished to the Upside Down by Eleven.
In volume 2, he wanted Max dead so he could break his main enemy back in Hawkins. At the end of season 4, Hawkins gang burnt Vecna but they failed to kill him and they couldn't stop the Upside Down from breaching Indiana through a series of earthquakes. Max died for a few moments before Eleven revived her and she went into a coma. Now a new dark theory suggests that Max could turn evil in season 5.
At the end of season 4, Eleven tried to enter Max's mind and found it blank. This has opened up the path for a Trojan horse. As Vecna survived, he can project his mind into the void just like Eleven did. This way Vecna can trick Eleven and co and put his mind into Max's body, coming back to life and duping the heroes. It would be just like the "piggyback" trick Eleven used to fight him. Thus Max could be used as a puppet—much like Billy—and become the new villain of season five.
A user commented on this theory on Reddit,
"This is pretty close to what I think is gonna happen to Max, except a) I think Vecna/One still has her soul and is torturing her, meaning that part of the plot will be trying to find a way to bring her back, and b) it's gonna be Vecna/One himself who possesses Max. That said, your idea, that the "old" Max is well and truly gone and that her body's now nothing more than a vessel for season 5's villain, is a whole lot darker than mine."
This would give Vecna human form once again, possessing Max the same way he did Will. In season 2, Will was possessed and acted as an extension of the Flayer. In Max's case, because Vecna has more power, he can fake being Max a bit better, all before turning on the team.
Ultimately, an evil Max using Vecna's powers could be the big twist in season 5, forcing the kids to fight one of their own.