Maybe Lucius deages Snape out of jealousy. Snape was always a bit of a rival in Voldemort's eyes. In his deaged state, Snape might form unexpected friendships with students at Hogwarts. He could befriend Hermione, for example, who would be intrigued by this strange new version of Snape. Meanwhile, Lucius has to keep his actions a secret from Voldemort, as Voldemort might not be happy with his meddling in Snape's life.
It could be that Lucius accidentally deages Snape while trying out a new, experimental spell. Then, the story could revolve around how Snape has to deal with being 16 again in a world that has moved on. He might struggle with his memories of the future and how to interact with people who knew him as an adult. For example, he'd have to face his former students who are now his peers, and that could create some really interesting and complex situations in the fanfiction.
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.
Snape's deaged personality could also be more impulsive. Without the restraint of his adult self, he might act on his emotions more quickly. For example, if he felt wronged, he might lash out immediately instead of plotting a long - term revenge like his older self would do. Also, he could be more curious about the world around him, eager to explore magic in a new way that his older self had forgotten due to years of being jaded.
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.
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.
Well, perhaps in the fanfiction, they are all entered into a magical tournament. Harry, being the young and brave one, Snape with his vast knowledge of magic, and Lucius with his family's resources. At first, they are rivals in the tournament but then realize that there is an outside force trying to manipulate the outcome for evil purposes. So they join forces. Or it could be that a powerful dark magic spell has been cast over Hogwarts, and only the combined magic of Harry, Snape and Lucius can break it. They start off distrustful but as they work together, they form an unlikely alliance.
Snape and Lucius can be presented in various ways in fanfiction. One common portrayal is that Lucius, being from a wealthy and influential pure - blood family, may look down on Snape at first due to Snape's half - blood status. But as they grow older, they find common ground in their service to Voldemort (although Snape has his own hidden motives). Their relationship might be full of intrigue and hidden agendas.
One plot idea could be that in a world where dark forces are targeting Hermione for her intelligence and her role in the fight against Voldemort. Snape, due to his complex relationship with the Order of the Phoenix, and Lucius, perhaps having a change of heart after seeing the true brutality of Voldemort's regime, team up to protect her. They could hide her in an old, magically protected manor that has been in Lucius's family for generations. Snape uses his skills in potions to create protective elixirs for her, while Lucius uses his knowledge of dark magic to set up wards that only they can pass through.
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.