Perhaps it could be a story about a young Snape. His family situation was not great, and if he was ill or had some sort of accident as a young child, he had to be put in diapers. This could be a very emotional and in - depth look at his early life, his relationship with his parents, and how these early experiences influenced the man he became at Hogwarts.
A creative reason could be a magical prank gone too far. The Marauders or some other mischievous students play a prank on Snape using a new, untested spell. The spell backfires and Snape ends up in a state where he needs to be diapered. This would lead to a story of revenge, forgiveness, or just chaos at Hogwarts.
The sister could have her own love story intertwined with the events of the wizarding world. She might fall in love with a Gryffindor, which would be a huge scandal in the eyes of some. This relationship could be a source of tension, not only within her family but also in the wider wizarding community. And as Severus tries to protect his sister from the dangers of the wizarding war, her relationship could add an extra layer of complexity to his already difficult situation.
One possible plotline could be that Snape discovers a hidden weakness of Voldemort. Maybe through his years of spying, he finds out that a certain spell, when combined with an ancient magic artifact, can destroy Voldemort. So he devises a plan to get hold of the artifact and uses the spell at a crucial moment when Voldemort is off - guard, perhaps during a major battle when Voldemort is focused on other opponents.
One possible plotline could be the daughter's struggle with her father's complex reputation at Hogwarts. She might face prejudice from students who remember Snape's dark past. Another could be her discovery of her father's true love for her mother, which was hidden for so long. And she could also have a special talent in potions, just like her father, and use it to outshine others in her classes.
Well, a plotline might involve Snape initially being in denial about the pregnancy. As it progresses, he could start to form an unexpected bond with the unborn child. There could also be a plot where other characters find out and either support him or try to use it against him. Maybe Voldemort discovers it and tries to manipulate Snape in a new way because of this situation.
One possible plotline could be that Harry discovers a plot against Snape before it's too late. Maybe he overhears some Death Eaters planning to ambush Snape while he's on a solitary mission. Harry then uses his invisibility cloak and magic skills he's learned over the years to outwit the Death Eaters and save Snape. This would show Harry's growth as a wizard and his ability to set aside past differences with Snape.
A plot could involve Snape's deaging being the result of a dark magic experiment gone wrong. Harry and Ginny initially suspect foul play but soon realize that Snape is truly a child in mind and body. They decide to protect him from those who might want to use his situation for their own gains. As they do so, Snape begins to form a bond with them, especially with Ginny, who he sees as a sister - like figure. Harry, meanwhile, has to deal with his own mixed feelings towards this new Snape, as he is constantly reminded of the old, often - hated Snape, yet also sees the potential for a new beginning.
One possible plotline could be that Harry travels back in time. In this scenario, he stumbles upon a young Severus Snape being bullied by some older kids. Harry, with his knowledge of Snape's future importance and his own sense of justice, steps in to save Snape. This could then lead to a change in their relationship dynamic. Instead of the animosity that exists in the original story, Snape might view Harry in a different light from the start, which could potentially change the whole course of events at Hogwarts in the future.
There could be a plot where Snape's muteness is a secret that he tries to hide from everyone at Hogwarts. The story could be about how he manages to teach Potions and be a double agent for Dumbledore all while keeping this secret. Or perhaps a plot where his muteness is a magical experiment gone wrong, and the story is about his journey to reverse it. Along the way, he might discover new aspects of magic and his own identity that he never knew before.
Well, a plot could be that Snape adopts Harry due to some unforeseen circumstances. For example, Dumbledore in his infinite wisdom decides that Snape is the best person to protect Harry. Snape initially hates the idea but as he spends time with Harry, he starts to see Lily in him, not just in appearance but in his kind heart. This makes Snape more protective. Harry, for his part, is initially scared of Snape but soon realizes that Snape is actually trying to help him grow into a powerful wizard. They start to have a mutual understanding, and Snape helps Harry with his studies and also with dealing with the trauma of his past.