The plot could involve Snape seeing the abuse Harry endures at the Dursleys and feeling a sudden pang of empathy. He decides to adopt Harry and bring him into the wizarding world in a more controlled manner. Snape would introduce Harry to the darker aspects of magic early on, but also teach him how to use it for good. There could be a lot of tension as Harry tries to figure out Snape's motives, and Snape has to deal with the fact that he now has a young boy to look after, which is completely different from his usual solitary life.
In this fan fiction, Snape might adopt Harry because he realizes that Harry has a lot of potential that is being stifled at the Dursleys. He could take Harry to a secret location and train him not only in magic but also in self - defense and survival skills. Snape would have to deal with his own complex feelings towards Harry, as he has both resentment and a sense of obligation. And Harry would have to adjust to living with Snape, who is not exactly the warmest person. It could be a story of growth and understanding between the two.
One big challenge is their pre - existing relationship. They've had a lot of conflicts in school. Snape has been rather harsh on Harry, so they need to overcome that animosity. Also, Snape has to deal with Harry's trauma from the Dursleys. He needs to help Harry heal and grow in a positive way, which is not easy for someone as emotionally reserved as Snape.
Well, he might be quite reluctant at first. Snape has always had a rather complex relationship with Harry. He might see it as a huge burden and be full of doubts. After all, he has his own grudges and past issues with James Potter, Harry's father. But on the other hand, he also has a sense of duty towards Harry as he was Lily's son. So it's a mix of emotions that he has to deal with.
Harry might become more reserved and cautious at first. Growing up with Snape, who is not as openly affectionate as others, would make him more self - reliant. He'd also likely become more skilled in magic quicker, as Snape is a very talented wizard.
In this fan fiction, Snape could save Harry from Voldemort's followers who are after him. The blood adoption might be a way to give Harry a new family connection. Snape, having complex feelings for Harry's mother, might see this as a way to fulfill a promise or a way to atone for past actions. He could then teach Harry some of his more advanced potions knowledge and help him develop his occlumency skills better.
One possible plot twist could be that Voldenort adopts Harry with the intention of turning him into a dark wizard, but Harry's inherent goodness starts to change Voldenort instead. Maybe Harry shows Voldenort kindness that he has never experienced before, leading to a shift in Voldenort's character.
Maybe Severus discovers some dark magic plot that involves the Dursleys harming Harry in a more serious way. So, he intervenes just in time. He could use his skills in potions and magic to subdue the Dursleys if they resist, and then take Harry to a safe place, like his own secret hideout or a friendly wizard's home.
The plot twist could be related to Snape's true motives. He might be under an Unbreakable Vow to rescue Harry from the Dursleys. In the process, Harry realizes that Snape is not as bad as he seems. This could lead to a complete change in their relationship. Harry could start to trust Snape, and Snape, in turn, could teach Harry some unique magic that he has never learned before, which is crucial for the upcoming battle against Voldemort.
Well, perhaps in the fan fiction, Severus saves Harry from the Dursleys but then has to deal with the consequences of going against Dumbledore's plan. Dumbledore might have had a reason for leaving Harry with the Dursleys, and Severus' actions could set off a chain of events that changes the whole wizarding world. Also, Severus could find out that the Dursleys were part of a much larger conspiracy against the wizarding world, and by saving Harry, he uncovers this plot.
Perhaps Severus, while rescuing Harry, stumbles upon a hidden connection between the Dursleys and the Death Eaters. This would add a whole new layer of danger and mystery. He might have to fight off some of his former allies to get Harry to safety. It could also lead to a change in Severus' perception of Harry as he realizes the boy has been in more danger all along than just from Voldemort.
One interesting plot point could be Snape's internal struggle as he decides to save Harry. He has his own complex history with Harry's parents, so having to put aside his grudges and protect the boy adds a lot of depth. Another might be how Harry reacts to being adopted by Snape. Does he initially resist or is he secretly glad to have someone who truly cares in this new version of the story? Also, the way Snape has to hide this new relationship from the rest of the wizarding world to protect Harry from potential enemies or those who might not understand his motives.