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.
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.
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.
Sure. One great Vecna fan fiction is 'Vecna's Redemption' which explores a different side of Vecna. It shows his past and how he might have been influenced by certain events in the Upside Down. Another one is 'Vecna's Allies', which delves into the possible allies he could have had or might acquire in the future.
There are many possibilities. Maybe a fanfic where Will discovers Vecna's weakness and uses it to save the town. Or one where they have a heart-to-heart conversation and understand each other better.