One interesting deaged Snape and Harry fanfiction could be about Snape being deaged to Harry's age and them having to work together on a special project at Hogwarts. They might start to see each other in a new light, away from their usual teacher - student dynamic.
You can try fanfiction.net. It has a large collection of Harry Potter fanfictions, and there's a good chance you'll find some great deaged Snape and Harry stories there. Just use the search function and type in relevant keywords like 'deaged Snape Harry' and browse through the results.
It's popular because it creates an interesting dynamic. Snape has always had a complex relationship with Harry in the original series. When Harry is deaged, it gives Snape a chance to start anew and perhaps be a different kind of influence. It also allows for exploring Snape's more nurturing side which is a contrast to his usual cold persona.
In deaged Snape fanfiction, the theme of redemption can be quite prominent. It gives Snape a chance to rewrite some of his past actions or at least be seen in a new light. There's also the theme of identity crisis. He's not the same old Snape, and he has to figure out where he fits now. Additionally, the theme of adaptation is common. He has to adapt to his new physical state, new social situations, and new magical abilities or limitations that come with being deaged.
There's also 'Hermione's Second Childhood with Snape'. It's a unique take on the concept. The story shows how Hermione's deaging affects not only her relationship with Snape but also how she views the wizarding world all over again. The author creates a lot of heart - warming and sometimes tense moments between the two characters as they navigate this strange new situation.
In the fanfiction world, if Snape was deaged to 16 by a potion, he might have a whole new set of challenges. Firstly, his relationships would change. He'd probably have a different dynamic with his peers at Hogwarts. His knowledge and skills would still be there, but his physical and emotional state would be that of a 16 - year - old. He might struggle to be taken seriously by those who knew him as an adult, and his interactions with students could become more complex as he'd be closer in age to them.
I'm not sure of a specific fanfiction with this exact plot, but you could try searching on popular fanfiction platforms like Archive of Our Own. Use the tags 'Snape', 'deaged', 'Lucius' and '16' to narrow down your search.
One possible plot could be Snape having to re - experience his Hogwarts years at 16. He might struggle with his old memories while also dealing with the current Hogwarts environment. For example, he could have a different perspective on his relationships with other characters like Lily. Maybe he tries to change things that he regretted in his original past.
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.
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.