Well, it could be that a magical spell or some devious plot made others believe Harry was the father. In the fanfiction, Harry might go on a journey to uncover the truth. He could encounter various characters who either help or hinder his quest to clear his name and expose the real father situation. This could involve exploring different magical realms or using his wits and skills to get to the bottom of things.
Maybe in the fanfiction, there has been some sort of misunderstanding or false accusation that made people initially think Harry was the father. Then the story could be about him proving his innocence and finding out who the real father is.
Maybe the fanfiction could focus on Harry as a father dealing with the pressure of his kids being in different Hogwarts houses. For example, if one of his kids was sorted into Slytherin, he would have to overcome his own prejudices from his school days and support his child wholeheartedly. He could also teach them about the importance of unity between the houses.
In this fanfiction, it could start with an experiment in the magical research of both universes. Harry Potter, while exploring new magic, accidentally creates a portal to the world of Kotomine. Finding the young Kotomine in a difficult situation, Harry decides to stay and help him. As time passes, Harry imparts his knowledge of friendship, love, and the power of good magic to Kotomine. Their relationship grows, and they face various threats together, including enemies from both the Harry Potter and Fate universes who see their relationship as a threat to the established order. Harry's role as a father figure would be central to Kotomine's development, and they would embark on a journey of self - discovery and adventure.
One possible plot could be that a character, perhaps an unknown wizard, is revealed to be the 'Father of Monsters' in the Harry Potter universe. He could be creating new and dangerous creatures secretly, and Harry and his friends stumble upon this mystery during one of their adventures. Maybe these monsters are being used to disrupt the wizarding world's peace, and it's up to Harry to stop this 'Father of Monsters' and his evil plans.
A possible plot might involve the 'father of vampires' being a key figure in a prophecy that Harry discovers. The vampire has the power to either save or destroy the wizarding world. It turns out that he has his own motives and desires that are not entirely clear at first. He may be trying to gain control over the vampire population in the wizarding world, which has been growing out of control. Harry and his friends have to figure out whether to trust him or not as they try to prevent a war between wizards and vampires that could be catastrophic for both worlds. They also learn about the true origins of vampires in the Harry Potter universe through their interactions with the 'father of vampires'.
One possible plot could be that Harry has a past that he's unaware of, and it turns out he has a son in another dimension, which is Tsuna. Harry then embarks on a journey to Tsuna's world. Along the way, he meets different characters from Tsuna's story. He might have to learn about Tsuna's world and its rules. He could also teach Tsuna some of his magic, and together they face challenges like the threats from the mafia in Tsuna's world. Harry would be trying to build a relationship with Tsuna, who is initially confused but gradually warms up to having a father.
In this fanfiction, perhaps there's a spell or some dark magic at play that has misled Harry all his life about his father. It could lead to a whole new adventure as he tries to find out the truth. He might have to face some old enemies of his real father or make new allies in the process of uncovering his true heritage.
Perhaps the fanfiction could be set in a magical world. Harry, having some special powers, has to protect Rose from various threats. He teaches her how to use magic or special skills. Along the way, their relationship deepens, and they form an unbreakable bond. There could be adventures and mysteries that they solve together as father and daughter.
Well, it could be about Harry Hook learning important life lessons from his father. Maybe his father teaches him how to be a better pirate, like showing him secret navigation techniques or how to handle a crew.
In this fanfiction, it could start with a prophecy that links the two. Aragorn, who is used to prophecies from his own world, takes it seriously. He travels to find Harry and bring him to Middle - earth for training. Along the way, they bond over their shared experiences of loss (Aragorn's father and Harry's parents). Once in Middle - earth, Harry has to learn the ways of the land while also trying to figure out how his magic can be used in this new environment. Their relationship would be one of mutual respect and growth as they both learn from each other.
The plot could involve a mystery around Tom's birth. Harry might suspect foul play or some sort of prophecy that led to Tom being his son. As he delves deeper into the mystery, he uncovers secrets about his own family and the forces that brought Tom into his life. This could lead to a series of adventures where Harry and Tom bond while trying to solve the mystery.