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.
Perhaps the deaged Snape has some latent magical abilities that are different from his adult self. Harry and Ginny notice this and try to help him develop these powers in a positive way. This could lead to Snape being involved in some new adventures at Hogwarts. He might end up befriending students he previously had a difficult relationship with. For example, Neville could see this new Snape as a friend and help him with Herbology. Harry and Ginny's relationship could also be explored further in the context of their shared responsibility for Snape, with their love for each other growing as they face challenges together while taking care of Snape.
The relationship between Harry and Snape would change drastically. Normally, they had a very strained relationship in the original story. But with Snape deaged, Harry would see him more as a vulnerable being in need of help. He might feel a sense of protectiveness towards Snape, which is a complete turnaround from his previous feelings. As for Ginny, she would probably approach Snape with an open mind. Her relationship with Snape would be more of a nurturing one, like taking care of a lost child. And between Harry and Ginny, their relationship could be strengthened as they share this unique experience of looking after Snape.
One possible plotline could be that Harry breaks a major Hogwarts rule, like sneaking into the Forbidden Forest. Snape catches him and decides that a severe punishment is in order, which in this case is spanking. Another could be that Harry fails a very important Potions test repeatedly, and Snape, frustrated with his lack of progress, resorts to this extreme form of discipline. However, these are all just fan - created ideas and not part of the official story.
One interesting plotline could be Severus having to take care of a deaged Harry. He might struggle with his initial dislike for Harry but gradually grow to understand him better in this new situation. For example, he could teach the young Harry some basic potions skills, which would be a unique bonding experience.
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.
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.
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.
One possible plotline could be that Severus Snape discovers he has royal blood. He then has to navigate the complex world of royalty while still dealing with his duties at Hogwarts. Maybe he uses his new - found status to gain more influence in the wizarding world, but also has to face those who are jealous or suspicious of his sudden rise.
One plotline could be that the de - aged Voldemort, now in a more innocent state, tries to make amends for his past actions towards Harry. He might use his knowledge of magic to protect Harry from other threats in the wizarding world. Another could be that Harry initially resists the idea of Voldemort as a father - figure but slowly comes to understand that this new Voldemort is different. And then there's the plotline where the wizarding world is in chaos because of this strange relationship, with some people believing it's a dark plot while others see it as a chance for redemption.
Maybe they are trapped in the house due to some magical curse. While they are alone, they are forced to confront their past issues. Harry might question Snape about his relationship with his mother, and Snape could finally open up about his true feelings and the sacrifices he made. This would be a very emotional and character - developing plotline.
One possible plotline could be that Snape initially hides his relationship to Harry out of fear or a sense of duty. He might secretly protect Harry more than anyone realizes. For example, his constant bickering with Harry could be a way to hide his true feelings. Another aspect could be how Harry discovers the truth. Maybe he finds some old letters or a hidden memory in Snape's Pensieve. This revelation would completely change Harry's perception of Snape and his own identity. It could also lead to a complex relationship between Harry and his mother's memory, as he would have to reconcile Snape's role in her life with his newfound knowledge of Snape as his father.