Well, perhaps in a fanfiction, Harry somehow travels back in time and meets a younger Tom Riddle. He tries to prevent Tom from turning into Voldemort. In the process, they get to know each other on a deeper level and fall in love. Or there could be a story where they are both cursed, and the only way to break the curse is by sharing a true love's kiss, which then leads to their relationship.
One idea could be that Harry and Tom meet in an alternate universe where they are not enemies. They discover they have a shared love for magic theory and start to study together, gradually developing feelings for each other. Another plot could be that they are forced to work together on a dangerous mission. Through facing hardships side by side, they realize their connection.
A possible plot for Harry and Tom mate fanfiction is that they are both sent on a special mission by Dumbledore. Along the way, they have to rely on each other's strengths. Tom might use his intelligence and cunning, while Harry uses his courage and loyalty. They could also encounter magical creatures that they have to work together to defeat. This would force them to put aside their differences and form a bond. And as they complete the mission, they realize that they can learn a lot from each other.
One idea could be that they discover their brotherhood through a magical accident. Maybe a spell goes wrong and reveals their hidden family connection. Then they have to deal with the shock and also how it will change their relationships with others at school.
One idea could be a crossover where the magic from the Harry Potter world is introduced to the vampires in Diabolik Lovers. For example, a Hogwarts student could accidentally end up in the Diabolik Lovers' world and use spells to both defend themselves from the vampires' advances and also try to find a way back home.
In a more complex plot, Oberon falls in love with a character in the Harry Potter world. Harry becomes involved as he tries to help Oberon navigate the human - or rather wizard - emotions and social norms. This could involve dealing with other magical beings in the Potter universe who are suspicious of Oberon's presence and intentions.
Perhaps the locket contains a part of Tom's soul that is still somewhat 'good'. Harry accidentally releases this part while investigating the locket. This 'good' part of Tom then tries to help Harry defeat the remaining evil parts of Voldemort, leading to a very strange alliance between Harry and this new aspect of Tom.
A more complex plot might be where both Harry and Tom are sent back in time to a point where the Hogwarts founders are still alive. They get involved in the founding events of Hogwarts, and Harry has to stop Tom from influencing the founders in a way that would make Hogwarts more exclusive and dark - sided. Harry could try to work with the founders to see the potential danger in Tom even at a young age.
One idea could be that they work together to solve a mystery in the wizarding world that neither could solve alone. Maybe it's about a series of strange disappearances in Hogwarts that are linked to an ancient curse.
One idea could be a time - travel fanfic where Harry goes back in time and tries to prevent Tom Riddle from becoming Voldemort. He could try to befriend the young Tom and show him a different path. Another could be a fanfic where Harry and Tom are somehow trapped in an alternate dimension together and have to work together to escape, all while Harry tries to understand what made Tom turn so evil. A third idea is a fanfic where Tom Riddle actually has a change of heart early on due to some magical accident that makes him experience a different kind of love and kindness, and Harry has to deal with this new Tom in Hogwarts.
One plot idea could be that Fem Harry, with her unique perspective as a female in a male - dominated wizarding world, catches Tom Riddle's attention in Hogwarts. He's intrigued by her different way of approaching magic. Maybe she's more empathetic and uses magic in a way that he's never seen before, leading to a complex relationship where he both admires and is jealous of her power.
One plot idea could be a time - travel scenario where Harry goes back in time and meets a young Tom Riddle. He tries to change Tom's path to prevent him from becoming Voldemort. Another could be a situation where they are magically bound together and have to learn to co - exist and understand each other's motives.