The plot could involve their family secrets. Perhaps there was some sort of hidden connection between their families that was kept from them. As they explore this newfound brotherhood, they face challenges. For example, they might have to deal with people from both of their worlds who don't approve of their relationship. They could also learn from each other, with Harry teaching Tsuna about magic and Tsuna showing Harry some of his street - smart skills.
Well, one possible plot could be that they discover their brotherhood in a mysterious way. Maybe they start having strange dreams or visions that lead them to find each other. Then they could combine their powers, Harry's magic from the wizarding world and Tsuna's combat skills from his own world, to fight a common enemy.
A possible plot could be that they are separated at birth due to some dark forces. As they grow up in their respective worlds, they start to notice strange similarities in their abilities and experiences. When they finally meet, they have to figure out how to work together. Along the way, they make new friends and allies from both worlds. They might also have to face an ancient evil that threatens both of their worlds, and only by combining their unique strengths can they hope to defeat it. This would involve a lot of character development for both Harry and Tsuna as they adjust to having a brother and learn to trust each other.
Yes, there are likely some fanfictions out there with such a concept. Fanfiction writers often come up with creative and unique crossovers or family relationship ideas. You can try searching on popular fanfiction websites like Archive of Our Own or Fanfiction.net to find such works.
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 Harry adopts Tsuna because he senses a great power within Tsuna that needs to be guided. Harry could use his experience with the Dursleys and Hogwarts to give Tsuna a stable and loving home. He might train Tsuna in both magic and combat, and together they could face new threats that are a combination of the threats from Tsuna's world and the wizarding world, like dark wizards teaming up with Tsuna's enemies.
One possibility could be that Harry, with his knowledge of magic and experience in fighting dark forces, takes Tsuna under his wing in the magical world. He could teach Tsuna about different spells and how to handle magical creatures, which would be a new and exciting experience for Tsuna who is used to his own world in 'Katekyo Hitman Reborn!'.
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.
The plot could also involve Harry and Bella's joint adventures. They could team up against common enemies that might emerge from the combination of the two fictional universes. Harry might use his knowledge of magic and Bella could use her understanding of the vampire - werewolf politics in Forks. They could also face challenges within their family, like how their parents react to the strange things happening around them.
One possible plot could be that they discover their brotherhood through some sort of magical or ninja - related mystery. Maybe there's a hidden prophecy or an ancient artifact that reveals their connection. Then they have to team up to face a common enemy that threatens both their worlds, like a dark wizard - ninja alliance.
Well, the plot could revolve around family secrets. Maybe Harry discovers that their family has some sort of special heritage that ties into both the world he comes from and Haruhi's world. They could go on a journey together to uncover the truth about their family, all while Haruhi is still dealing with her school life and the Host Club. Harry might have to protect Haruhi from some threats that are related to this family secret.
One possible plot could be that Harry, being Voldemort's little brother, has been hidden away from the wizarding world by some powerful magic. As he grows up, he starts to discover his true identity and his connection to Voldemort. This discovery throws the wizarding world into chaos as they don't know whether to trust Harry or see him as a threat.
One possible plot could be that Harry discovers this long - lost family connection after some mysterious event. Maybe he finds a hidden letter or an old family heirloom that reveals the truth. This discovery could throw him into an identity crisis as he has to come to terms with being related to a Death Eater. He might then try to find out more about his family's past and his connection to Bellatrix, which could lead him into dangerous situations as he delves deeper into the dark secrets of his family.