One interesting plot twist could be that Snape's blood adoption gives Harry some of Snape's memories. This would allow Harry to see Snape's true motives throughout the years, not just the ones Dumbledore had revealed. Another could be that the adoption gives Harry a new magical ability related to Snape's potions expertise, like the ability to brew extremely powerful potions without much effort.
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.
The plot could take a really interesting turn when it comes to Voldemort. Since Snape is raising Harry, he might train Harry in a way that Voldemort doesn't expect. Maybe Harry would be more of a spy in the dark side's camp. Snape could also use his position to mislead Voldemort about Harry's true abilities and intentions. This would create a whole new set of challenges and power dynamics in the fight against the Dark Lord. Harry's loyalties would be constantly tested, and his relationship with Snape would be even more crucial to the story.
Well, it could be that Bellatrix adopts Hermione thinking she can use her to get to Harry Potter. But as time goes on, Hermione manages to turn the tables and uses Bellatrix's own desires against her. She convinces Bellatrix that she can lead her to greater power if she turns against Voldemort. This not only creates a rift between Bellatrix and her dark lord but also sets off a chain of events that no one could have predicted.
In some fanfics, a plot twist could be that Severus was actually protecting Hermione all along under Dumbledore's secret orders, and Sirius discovers this and has to re - evaluate his hatred for Severus. This completely changes the dynamic between the three.
One possible plot twist could be that Harry discovers he has a long - lost sibling. This would completely change his understanding of his family and his place in the wizarding world. It would also have a huge impact on the relationships he has with other characters, like Dumbledore who might have been hiding this truth from him.
Imagine if the Marauder's Map could show the layout of Narnia. Harry and his friends could use it to navigate through Narnia, which would be a huge advantage. But then, there could be a twist where the map starts to malfunction in Narnia because the magic there is different. And a character like Mr. Tumnus could be revealed to have some connection to the wizarding world in Harry Potter. Maybe he was once a wizard who was exiled to Narnia and lost his magic, but regains it when the two worlds start to interact.
The blood adoption might make Harry more introspective. He has always been a bit impulsive, but with Snape's influence through the adoption, he may start to think more before acting. For example, he might analyze situations from different angles like Snape used to.
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.
One plot could be that Snape, under some dark magic influence, forces Harry to accompany him on a dangerous mission to retrieve a lost magical artifact. Snape believes Harry has some hidden power that can help. Another could be Snape forcing Harry to study a very advanced and dangerous form of magic, thinking it will protect Harry in the long run. And there could also be a plot where Snape forces Harry to enter a magically sealed area to face some long - forgotten threat that only Harry can potentially defeat.
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.