In a 'Harry Potter x Reborn fanfiction', Harry could be reborn in a time before Tsuna is aware of Reborn's presence. Harry, with his knowledge of magic and his experiences from his past life, could start to prepare for the chaos that he knows will come when Tsuna gets involved with the mafia world. He could secretly start to train himself in the skills needed in the 'Reborn' world, like using his magic to enhance his physical abilities. He might also try to find ways to combine spells with the unique abilities in the 'Reborn' world, such as using the Imperio curse to control an enemy while using the Flames of Dying Will to boost his own power.
One idea could be that Harry is reborn into the world of 'Reborn' with his magical abilities intact. He could use magic to solve the mysteries and challenges in the 'Reborn' world, like helping Tsuna in a unique way. Another plot could involve the magic of the 'Harry Potter' universe somehow getting intertwined with the flames in 'Reborn'.
One idea could be that the main character from Reborn is reborn into the Harry Potter world as a wizard with his unique skills. He could use his combat abilities from Reborn to face the dark wizards in the wizarding world. Another could be that a character from Harry Potter gets reborn in the Reborn universe and has to adapt to the different rules and powers there.
One idea could be being reborn as Harry's long - lost twin. You could then have a whole new dynamic where you both share the burden of being the Boy Who Lived, but also have your own unique abilities and personalities. Another plot could be being reborn as a Slytherin who tries to change the house's reputation from the inside. You might start off being shunned by the other Gryffindors and friends of Harry, but slowly prove that not all Slytherins are bad.
One interesting plot could be a character reborn with memories of the future. For example, a minor character who remembers Voldemort's rise and fall and tries to change things in a different way. They might try to prevent certain deaths or change the course of the wizarding wars.
Well, a plot might involve Harry - now James - trying to change the fate of his family. He could focus on protecting Lily in a different way than James did in the original story. Maybe he'd use his knowledge of magic from his past life to create new spells or defenses. Also, his relationship with Snape could be very different. Since he was Harry before, he might understand Snape's feelings better and try to make amends or change the animosity between them.
Perhaps he could use his phoenix form to travel to places that are otherwise inaccessible. He could fly to the Forbidden Forest's most secret corners and discover new magical creatures or ancient magic. Also, his rebirth as a phoenix might give him the power to communicate with other phoenixes, and they could pass on their ancient wisdom to him.
In this fanfiction concept, it might be interesting if Harry, as a Targaryen, is born with the ability to communicate with dragons in a more profound way than the typical Targaryen. This could be because of his past life experiences with magical creatures. He could use this connection to his advantage, uniting the different factions in Westeros. Also, his memories of the Sorting Hat might influence how he views the different houses or families in Westeros, and he could strive to create a more inclusive society.
One idea could be being reborn as a muggle - born wizard who has a unique ability to communicate with magical creatures in ways others can't. This could lead to all sorts of adventures in the Forbidden Forest and uncovering secrets about the creatures that help in the fight against Voldemort.
Well, perhaps in this fanfiction, Harry, now Tsuna, has visions of his past life at Hogwarts during his battles in the KHR world. These visions give him an edge as he can draw on the spells and strategies he used against Voldemort to fight his new enemies. Also, his relationship with Reborn could be really interesting. Reborn might be intrigued by Tsuna's strange bursts of knowledge and skills that seem to come out of nowhere, which are actually remnants of Harry's past magical training.
Well, perhaps a cool plot is that as a dragon, Harry discovers ancient dragon magic that has been long forgotten in the wizarding world. He then uses this magic to revive some of the lost magical creatures. Also, his interactions with other magical beings would be different. For example, he might have a unique bond with the centaurs as a dragon.
One idea could be that Harry Potter is reborn in the Naruto world as an Uzumaki. He could use his magical knowledge in unique ways, like using magic to enhance his chakra control. Maybe he creates new jutsu that combine magic and chakra, such as a spell - enhanced Rasengan.