Perhaps the girl is a huge fan of Harry Potter in the fanfiction world. When Harry discovers her, she's being bullied for her love of him and his story. So, he adopts her to protect her. He then shows her the real Hogwarts, not just the one she's read about. Along the way, they might face challenges from those who don't like the idea of Harry having an adopted daughter, but they overcome them together.
In this 'Harry Potter adopts a girl fanfiction', it could start with Harry feeling a strange connection to the girl. She might have some unique magical ability that is endangered. Harry, being the hero he is, adopts her to keep her safe. As she grows up, she discovers more about her powers with Harry's guidance. They also go on adventures together, exploring new parts of the wizarding world that Harry himself hasn't seen before.
One possible plot is that the girl is from a non - magical family but has a deep fascination with Harry's story. Harry discovers her and decides to adopt her as she has a natural inclination towards magic. He trains her privately at first, and as she gets better, he reveals her to the wizarding community. Along the way, they face some challenges from those who are against a Muggle - born being trained by Harry, but they overcome them together.
The girl might be a descendant of an old wizarding family that has fallen on hard times. Harry adopts her to revive the family's honor. She could have some special knowledge or heirloom that attracts Harry's attention and makes him decide to take her in.
Well, one possibility is that Harry realizes Fenrir's tragic backstory and decides to help him change. Maybe he uses magic to suppress Fenrir's werewolf side and tries to integrate him into the wizarding society. Harry could teach Fenrir about friendship and loyalty, and together they fight against the dark forces that once exploited Fenrir.
Maybe Aragorn adopts Harry because he sees the potential in him to be a great leader. He takes Harry under his wing to groom him into a leader who can unite different groups, just as Aragorn united the races in Middle - earth. Aragorn would expose Harry to the politics and power dynamics of Middle - earth, and Harry could use his own experiences of dealing with the wizarding world's power struggles to adapt and contribute. Also, Aragorn's strong moral compass would influence Harry to make more ethical decisions in his life.
One possible plot could be that Hawkgirl discovers Harry Potter in a dangerous situation in the wizarding world. Maybe he's being hunted by dark forces more intensively than in the original story. Hawkgirl, with her combat skills and unique abilities, decides to take Harry under her wing. She could teach him some of her fighting techniques, which would be a new aspect for Harry as he mainly focuses on magic in the original. This would lead to Harry having an edge in battles against the dark wizards and creatures.
In such a fanfiction, perhaps the plot could involve Harry seeing a child in a difficult situation, like an orphan in the wizarding world who has no family due to a dark wizard's attack. Harry, having been an orphan himself, feels a strong connection and decides to adopt the child. He then teaches the child about magic, love, and how to face the challenges in the wizarding world.
The plot could involve a prophecy that ties Tsuna and Harry together. Harry, being the hero he is, feels responsible for Tsuna. He adopts Tsuna and brings him to live in the wizarding world. Tsuna is amazed by all the magical things around him, from the moving portraits to the flying broomsticks. Harry teaches Tsuna about the dark forces in his world and how to fight them. Tsuna, with his own tenacity, starts to train hard. Along the way, they make new friends together and face many challenges that test their bond and their magical abilities.
Well, it could be that the kid has some special powers just like Harry. Harry sees himself in the kid and decides to take him under his wing. Maybe the kid is an orphan too, and Harry wants to give him a better life at Hogwarts, away from the cruel muggle world that didn't understand him when he was a kid.
Well, perhaps Harry could find Sasuke in a strange magical realm that connects the Naruto world and the wizarding world. Harry, being the kind - hearted person he is, decides to take Sasuke under his wing. He could introduce Sasuke to Hogwarts, and Sasuke might be amazed by the magic there. He would learn spells instead of ninjutsu, and they could have adventures together fighting against dark wizards who might be interested in Sasuke's special powers.
Well, perhaps Harry Potter arrives in Gotham City and is appalled by the state of it. He decides to adopt some of the children who are in difficult situations. As he does this, he uses his Hogwarts education and magical skills to protect them from the likes of the Penguin and other villains. He might even introduce some magic into Gotham's society in a very subtle way, like using spells to improve the living conditions in the slums where he and the adopted kids live.