It could be that there's a new dark wizard in the fanfic who is out to get all those who were once associated with Dumbledore. Snape, being one of Dumbledore's closest confidants, becomes a prime target. The kill attempt might be a part of a larger plot to take over Hogwarts and turn it into a stronghold for dark magic. And the story could revolve around Harry and his friends trying to stop this and save Snape.
Well, it could be that Snape is wrongly accused of a great crime in this fanfic, and in the heat of the moment, someone attempts to kill him. Maybe it's a misdirected act of revenge by someone who thinks Snape is behind a series of dark events at Hogwarts. But of course, the truth might be far more complex and intertwined with the overall politics and secrets of the wizarding world.
Perhaps in this fanfic, Voldemort has put a spell on Harry that makes him believe Snape is a threat to his life and the safety of his friends. So, Harry, in a moment of confusion and under the influence of dark magic, goes after Snape with the intention to kill. This would create a huge moral dilemma for Harry later when he discovers the truth about Snape's true loyalties.
Perhaps in the fanfic, Snape discovers a powerful and dangerous magic that he wants to use for what he believes is the greater good, but Harry sees it as a threat. Harry has always been wary of Snape, and this new discovery could be the tipping point. Also, if Snape were to make a deal with a dark force in order to gain knowledge to defeat Voldemort but Harry misinterprets his intentions, it could lead to Harry killing Snape.
It could be Bellatrix Lestrange. She was always a fanatic follower of Voldemort and hated those who she thought were against the Dark Lord. Since Snape was a double - agent, she might have discovered his true loyalties and decided to kill him in the fanfic.
One possible motive could be a misunderstanding. Maybe Snape is framed for something really bad, like a dark magic plot against Hogwarts. Harry, in his haste to protect the school and his friends, believes Snape is guilty and takes drastic action. Another reason could be if Snape accidentally endangers someone Harry loves dearly, like Ginny or Ron, and Harry's emotions get the better of him in the heat of the moment.
One possible plot twist could be that Snape is secretly protecting Harry not just because of his connection to Lily, but because he discovers a new prophecy related to Harry that could change the fate of the wizarding world. So, he takes Harry in to keep a closer eye on him and train him in a way Dumbledore couldn't.
In a possible plot, the magical bond of being a familiar could give Harry and Snape a new level of understanding of each other. Harry, as Snape's familiar, could gain insights into Snape's past and his true motives. Snape, in turn, would have to come to terms with having Harry so close in this new capacity. This could lead to a series of adventures where they work together in a different way than in the original series. For example, they might have to face Voldemort together with Harry using his new familiar - related powers and Snape using his potions and dark arts knowledge.
One possible plot could be that they discover they are brothers early on in Harry's time at Hogwarts. Snape then has to balance his duties as a teacher and his newfound family connection. He tries to protect Harry in secret from the threats within the school, like Malfoy and his cronies.
One possibility could be that Dumbledore arranges for Harry to live with Snape for protection. Snape, though reluctant at first, gradually discovers Harry's hidden strengths and they form an unlikely bond. Harry might learn more about his mother from Snape, which could lead to a deeper understanding between them.
Perhaps in the fanfic, there is some sort of magic that binds their fates together. Snape might discover this connection and, despite their past differences and his own reserved nature, he decides to propose to Harry as a way to accept this newfound bond. It could also involve Snape trying to make amends for his past actions towards Harry by offering this ultimate commitment.
One possible plot could be that Harry discovers he has a hidden connection to Sebastian Snape, perhaps a long - lost family relation. This revelation could lead him to question his identity and his place in the wizarding world.
In a 'Harry Potter confides in Snape' fanfiction, the plot might involve Harry being tormented by visions of the future. He can't share these with Hermione and Ron as they are too worried about him already. So, he turns to Snape. Snape, with his knowledge of the Dark Arts and Occlumency, helps Harry understand and control these visions. This could lead to a new bond between them, different from their usual tense relationship.