One possible plotline could be exploring the hidden family connections between them. Maybe Harry discovers that he has some distant family relation to Tom Riddle through a convoluted family tree. This could lead to a conflict within Harry as he tries to come to terms with having a relation to the dark wizard. And Severus Snape's family could be intertwined in this discovery, perhaps his family having some role in the past that led to the current situation.
Another idea is that a long - lost family heirloom is discovered that has a connection to all three families. This heirloom holds some powerful magic that can either save or destroy the wizarding world. Harry, Tom Riddle, and Severus Snape's families all have a claim to it. They have to work together (or in the case of Riddle, try to take it for his own dark purposes) to figure out the mystery behind the heirloom and its true power.
One possible plotline could be that they meet in their school days at Hogwarts. Tom, being the ambitious and sly student he is, notices Eileen's quiet strength. Maybe he tries to use her for some of his dark plans initially, but as time goes on, he starts to develop unexpected feelings for her. Their relationship could be filled with secrecy and internal conflict as Eileen might be torn between her own moral compass and her growing attraction to Tom.
Tom Riddle's family values are often associated with a thirst for power and a disregard for others. His family, being descendants of Salazar Slytherin, might have passed down a sense of superiority. In fanfiction, his family values could be shown as the root of his dark path. He may have been taught from a young age that pure - blood wizarding families are superior, leading him to view those without pure blood as inferior and worthy of subjugation.
In fanfiction, Tom Riddle and Severus Snape are often depicted in complex relationships. Riddle, as the future Voldemort, represents pure evil. Snape, on the other hand, is a more morally ambiguous character. Fanfictions might explore how Snape would react to the younger Riddle at Hogwarts. Maybe Snape would be one of the few to see the darkness in Riddle early on.
One common portrayal in fanfiction is that of Snape being suspicious of Tom Riddle when he was still a student at Hogwarts. Snape, with his sharp intellect, might notice things about Riddle that others overlook, like his unusual interest in certain dark spells or his ability to manipulate those around him.
A plotline could involve them going on adventures outside of the wizarding world. They could travel to different magical realms, encountering strange creatures and magic systems. Along the way, they learn more about each other's strengths and weaknesses. Harry might learn from Tom how to be more strategic, and Tom could learn from Harry about the importance of loyalty and friendship. Their friendship could be tested in these new and dangerous situations, but ultimately it could grow stronger.
One possibility is that Tom Riddle sees potential in Severus Snape early on. Maybe he notices Snape's talent for potions and dark magic. He could adopt Snape to groom him into a powerful ally in his dark plans. This might lead to Snape having a different path in the wizarding world, being more directly involved with Riddle's rise to power rather than his later double - agent role for Dumbledore.
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.
Well, perhaps in this fanfiction, Harry adopts Tom Riddle Jr after a time - travel incident. Harry realizes that by changing Tom's upbringing, he can prevent the rise of Voldemort. He takes Tom under his wing and shows him the value of family. Tom, growing up with Harry, starts to develop different values. He becomes interested in protecting the wizarding world rather than ruling it. This could lead to a completely different wizarding world where the threat of Voldemort never really existed.