A really unique plot could be Snape adopting Harry during a time when Voldemort is still a threat. Snape has to balance protecting Harry from Voldemort's followers and also from the prejudice within the wizarding community. He might train Harry in Occlumency more intensively, and in the process, they develop a deeper understanding of each other. Harry starts to see Snape not as the mean professor but as a complex person with his own traumas and reasons for his actions.
Snape's character changes in his relationship with others too. He becomes more involved with the Hogwarts community as he is now responsible for Harry. He might start to collaborate more with other teachers, like McGonagall, who he previously had a rather cold relationship with. His attitude towards the students also changes as he realizes that they are just kids trying to find their way, much like Harry. This is a big shift from his usual strict and often unforgiving self.
In this fanfiction, the story could start with Harry finding Snape in a very vulnerable state after the Battle of Hogwarts. Harry feels a deep sense of guilt for how Snape was treated throughout his life. So he decides to adopt him. At first, their relationship is awkward. Snape is still his old sarcastic self, but Harry is determined. As time passes, they discover they have a lot in common. They both lost their parents at a young age, and they both had to face great challenges. This newfound understanding leads to a transformation in their relationship. Harry helps Snape to come out of his shell of bitterness, and Snape teaches Harry more about the Dark Arts in a controlled and ethical way so that Harry can better protect the wizarding world in the future.
One interesting plot could be Severus having to deal with Harry's fame at Hogwarts while trying to keep their new family relationship a secret. It adds a layer of mystery and tension.
Well, one interesting plot could be Harry seeing Severus in a new light. He realizes that Severus was actually a complex and misunderstood character. So, he decides to adopt him to give him a new start in life. Another plot might involve Severus initially being resistant to the idea of being adopted by Harry, but over time, he warms up to the concept as he sees Harry's genuine kindness.
One interesting plot could be Snape raising Harry while secretly training him in advanced potions and dark arts defenses. Harry would grow up with a different perspective on Snape, seeing him as a strict but caring father figure. Another plot might involve Snape having to protect Harry from other dark wizards who suspect Harry's true potential. This could lead to some intense magical battles and Snape using his knowledge and skills to keep Harry safe.
The relationship would start off rocky. Snape would be resistant to Harry's kindness as he has always seen Harry as James Potter's son. But Harry's persistent efforts to understand and help Snape would gradually break down Snape's walls.
In such a fanfiction, Harry might see the potential in young Severus that others overlooked. He could protect Severus from the bullies at Hogwarts. Harry, having experienced his own share of hardships, would understand Severus's situation to some extent and try to give him a better childhood, perhaps even guiding him to use his magic in a more positive way.
A common plot is the 'what - if' scenario. For example, what if Snape had confessed his love to Lily before she chose James? This often leads to alternate timelines where Snape and Lily end up together, and the whole story of Harry Potter changes. Another plot is the secret relationship plot, where Snape has a hidden relationship with someone (could be an OC or another character) that is related to his unrequited love for Lily.
Some unique plots include a fanfic where Snape and Harry are magically bonded and can feel each other's emotions. There's also a story where Snape is turned into a cat and only Harry can understand him, leading to some really interesting interactions. A different one is a fanfiction where Harry discovers that Snape was actually protecting him all along in a much more elaborate and secret way than in the original books, and they have to team up to expose a greater conspiracy within the wizarding world.
Severus could become more confident. With Harry's support, he may not be as shy or withdrawn as in the original story. He might be more open to making friends outside of Slytherin.