A really interesting plot twist could be that Snape and Harry accidentally swap magical abilities for a short time. This would force them to understand each other on a whole new level. Another possibility is that the Potters, while alive, have some sort of dark secret that Snape uncovers when he adopts Harry. This could put Snape in a difficult position, as he has to decide whether to keep the secret or expose it for Harry's own good. And perhaps there could be a magical accident that makes Harry and Snape age - swap, which would lead to all kinds of comical and also very character - developing situations.
One possible plot twist could be that Snape discovers some hidden magic in Harry that he didn't expect. Maybe a power that's related to the Potters but also has a connection to Snape's own family. Another twist could be that the other Potter family members don't approve of Snape adopting Harry, leading to a family feud within the wizarding world. And perhaps, there could be a dark force that tries to use Snape's new relationship with Harry against them.
In this kind of fanfiction, a major plot twist could be that Snape's past with the Death Eaters comes back to haunt him in relation to Harry. Maybe some old Death Eater contacts try to manipulate Snape into using Harry for their own evil purposes. Also, it could be that Harry starts to develop some of Snape's traits, which causes a stir among the Hogwarts students. Or there could be a plot twist where it's revealed that there was a prophecy long ago that predicted Snape's role in Harry's life, and this changes everyone's perception of their relationship.
Well, it could be really interesting. Snape might struggle with his feelings for Harry, given his past with James Potter. He'd have to balance his old grudges and his new role as a father figure. Harry would likely be confused at first, but also maybe a bit relieved to have someone, even Snape, looking out for him. And the wizarding world would be in for a shock to see Snape in such a different role.
One possible plot twist could be that Snape has a hidden agenda all along. Maybe he's been using Harry as a pawn in a bigger scheme related to the wizarding world's power struggle. Another could be that Harry discovers a long - lost family connection to Snape that changes their dynamic completely.
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, it could also be that Harry's Gryffindor nature starts to rub off on Snape in some ways. They might have arguments about how things should be done, but through these interactions, Snape might start to see the value in Harry's way of thinking. And Harry, on the other hand, might learn some of Snape's more cautious and calculating ways, which could help him in his future battles.
One possible plot twist could be that Snape blackmails Harry into joining a secret magical society at Hogwarts that Snape is leading. This society could be dedicated to protecting a powerful but dangerous magical artifact. Harry initially resists but realizes the importance of the cause as the story progresses.
One possible plot could be that Snape, despite his initial resentment towards James Potter (Harry's father), sees the vulnerability in little Harry. He decides to take him in out of a sense of duty to Lily (Harry's mother). Snape then has to adjust his life to care for a child. He teaches Harry potions at an early age and tries to protect him from the darker elements in the wizarding world. Harry grows up with a different perspective on Snape than in the original story.
One possible plot twist could be that Malfoy's family initially objects to the adoption but then slowly comes to accept Harry. For example, Lucius Malfoy might see potential benefits in having Harry Potter, the famous wizard, in their family. Another twist could be that Harry, while growing up with the Malfoys, starts to influence them to be less prejudiced. He might introduce them to his friends from Hogwarts like Ron and Hermione, which would be a huge culture shock for the Malfoy family.
In such a fanfiction, the plot could involve Snape taking on the role of a father figure to Harry and Draco. He might use his knowledge and skills to protect them from the various threats in the wizarding world. Snape could also teach them advanced magic, which they might not learn otherwise in Hogwarts. And there would likely be a lot of emotional growth for both Harry and Draco as they deal with their new family situation and their past traumas.
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.
A possible plot twist could be that Snape is actually the one who arranged for Harry to be sent to Azkaban, but for a noble reason. Maybe he knew that Harry was in danger from a more powerful dark force and Azkaban was the only place that could keep him safe temporarily. Another twist could be that the real culprit behind the crime Harry is accused of is someone very close to Snape, and Snape has to choose between protecting that person and helping Harry. It could also be that Snape discovers that Harry has a hidden power while in Azkaban that could change the fate of the wizarding world, and he now has to keep Harry alive at all costs.