In terms of the overall plot, it could lead to a race against time. The characters will need to find a way to exorcise Vecna from Will quickly. This might involve seeking out new knowledge, perhaps from an unexpected source. Maybe an old character from the past who has some knowledge about Vecna's possession returns to help. It also opens up the possibility for new alliances and betrayals within the group as they struggle with this new situation.
Well, one possibility is that Will starts to have visions that Vecna uses to manipulate the other characters. He might show them false images of their worst fears, leading to chaos within the group. For example, he could make Eleven see herself losing her powers and being unable to protect her friends.
One popular 'Harry gets possessed' fanfiction is 'The Possession of Harry Potter' on FanFiction.net. It has an interesting take on how the possession affects Harry's relationships with his friends. The author does a great job of building suspense throughout the story.
The theme of fear is often present in Vecna fan fiction. He is a terrifying character, and writers play with that aspect, showing how his presence instills fear in the characters in the story. Additionally, the theme of his connection to other entities in the Upside Down or in the real world is common. For example, how he might interact with other monsters or even with humans who have some sort of link to his power.
In some fanfictions, they might rewrite the possession scene completely. Instead of the typical way it was shown in the books, they could add new magical elements. Maybe Quirrell has some hidden magic that reacts to the possession, creating a more complex and engaging story.
Perhaps Vecna told Nancy his story to establish a connection with her or to confuse her. It might have been part of a larger plan to achieve his goals.
One possible scenario could be during a battle with a supervillain. Max might be overconfident and underestimate the villain's power. The villain could then land a powerful blow that leaves Max seriously injured. For example, if the villain has some sort of energy - based attack, it could hit Max and cause internal injuries. Another scenario could be an accident during a training session with his family. Maybe they are practicing a new superpower move and something goes wrong, resulting in Max getting hurt.