Well, one possibility is that they try to change Tom's fate. Since they know the dark path he'll take in the future, they might use their knowledge of magic and friendship to guide him towards the light. Maybe they introduce him to a different kind of magic, one that focuses on helping others rather than power for its own sake.
The plot could also involve a lot of conflict. Tom might resist their attempts at changing him. He could be very stubborn and set in his ways, even as a young boy. Harry and Hermione would then have to find new ways to reach him, like showing him the consequences of his future actions through time - travel magic or something similar.
Another aspect could be how the wizarding world reacts to this. Some might be against it, fearing that Tom will still turn bad and endanger everyone. But others might see it as a chance to prevent a great evil. Harry and Hermione would have to deal with the political and social pressures while also trying to raise Tom. They could face opposition from the Ministry of Magic, or from some pure - blood families who don't trust their motives.
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.
In a different take, the marriage law might be a magical experiment gone wrong. Hermione and Tom are joined in this unorthodox union. Hermione could use her vast knowledge of magic to try and find a way out, but in the process, she starts to see Tom in a different light. Tom, on the other hand, might be intrigued by Hermione's intellect and her different approach to magic compared to what he's used to in the wizarding world of his time. This could develop into a relationship where they both learn from each other and ultimately change the course of their fates.
One possibility could be a story of internal conflict for Harry. He might struggle with the dark heritage from Tom Riddle while trying to uphold the values of the wizarding world he has grown to love. He could constantly battle his own darker instincts that come from being Voldemort's (Tom Riddle) son. For example, he might have an in - built talent for Parseltongue that he has to learn to control and use for good instead of evil.
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.
The theme of trust is likely present. Fem Harry has to trust that she can influence Tom in a positive way. And for Tom, he has to learn to trust Fem Harry as his new family. There may also be a theme of identity. Tom has his own identity that is slowly being shaped into something dark, and Fem Harry is trying to reshape it. This fanfiction could also touch on the theme of sacrifice. Fem Harry may have to sacrifice some of her own peace or safety in order to take care of Tom and ensure he doesn't go down the wrong path.
One possible plot twist could be that Tom Riddle's dark tendencies start to resurface despite Hermione's positive influence. Maybe he begins to secretly manipulate those around him at Hogwarts while still maintaining a facade of being a reformed child. This could lead to a rift between Hermione and some of her friends who suspect Tom but don't understand why Hermione is so trusting.
One possible plot could be Hermione discovering the truth about her godfather being Tom Riddle and struggling with her own identity. She might be torn between her loyalty to her friends and the new - found family connection.
Perhaps in this fanfic, the apartment is a magical place that serves as a crossroads between different timelines. Tom Riddle stumbles into it while Hermione is already there studying some ancient magic. They are forced to co - exist in this small space. Tom is initially hostile, but Hermione uses her wits to keep him in check. As they interact more, they begin to understand each other's perspectives on magic, and a strange bond forms between them that could potentially change the course of the wizarding world.
Well, perhaps the plot could involve Hermione actually getting through to Tom Riddle in some way. Since she's so intelligent and empathetic, she might start to make him question his dark ways while being kidnapped. This could lead to a really interesting internal struggle within Tom. Also, there could be a twist where the kidnapping is part of a much larger plan that involves other dark wizards or witches, and Hermione has to figure out how to not only escape but also stop this bigger conspiracy. Or it could be that Hermione discovers a secret about Tom Riddle's past during her captivity that she can use to her advantage.
One possible plot could be that Harry, using his knowledge of love and friendship, tries to teach Tom about the power of positive emotions. He might show Tom how his own journey was shaped by the love of his friends and family, in contrast to Tom's more solitary and power - hungry path. Maybe Harry takes Tom to Hogwarts in a time - travel like scenario and shows him the true meaning of the houses, not just the pursuit of power within Slytherin.