One way could be through some sort of multiverse concept. Maybe there's a breach between the Naruto universe and the Harry Potter universe, and Tobirama somehow gets transported to the world of Harry Potter. Then, the story could build on how Tobirama adapts to this new world with its different magic and society.
It could be possible by reimagining the magic systems. Suppose the magic in both universes has a common origin deep in some ancient mythology. Tobirama, with his knowledge of chakra - based magic, could discover similarities with the magic in Harry Potter's world. For example, his ability to control water could be related to the water - based spells in Harry Potter. The story could then develop as Tobirama tries to find a way back home while also exploring this new magical world.
It would be extremely difficult. Harry Potter characters use magic wands and spells, while Vulcans are known for their logic and mind - melds. Maybe if there was some sort of magical - technological accident that merged the two universes, but it would be a stretch.
It's very difficult to see how it could be possible in a traditional sense. But if we stretch our imagination, perhaps in a fanfiction world, the author could create a parallel universe where Harry Potter is a big Eminem fan. Maybe Harry could rap some of Eminem's lyrics to defeat Voldemort or something equally absurd.
It would be very difficult. Maybe if you used some sort of magic - based multiverse theory where Harry Potter somehow ends up in the Marvel universe and takes on the skills or persona of Hawkeye. But that would require a lot of creative license.
One way it could work is by having Harry Potter be a secret admirer of Gomez Addams' unique style and way of life. Maybe Harry discovers some old wizarding texts that mention a connection to the Addams family's magic. He then becomes fascinated and starts to incorporate some of Gomez's traits into his own life, creating an interesting blend of the two worlds.
One way could be through a dimension - hopping concept. Maybe there's a rift in the fabric of the multiverse that allows Harry Potter to end up in the Star Wars universe. He could then interact with Luke Skywalker, and from there, a fanfiction could be built around their adventures together, with Harry using his magic in a world dominated by the Force.
One way it could work is by having Harry Potter be transported to the world of Noctis. He could use his magic skills there and face new challenges that are different from what he's used to in Hogwarts. For example, the magic in Noctis' world might be more elemental - based, and Harry has to adapt to use his wand magic in that context.
One way it could work is if there was a magical portal that connected the wizarding world to Spider - Man's universe. Harry could accidentally stumble through it and then get involved in Spider - Man's battles, using his magic to help out.
One way could be to give Harry the Geass power. So, he could use it to control the actions of Death Eaters or Voldemort himself. He might start to make more strategic and calculated moves like Lelouch, using his new power to outwit his enemies. Another aspect could be in the political side. Maybe the wizarding world becomes more like Lelouch's world with different factions vying for power and Harry has to play a political game as well as fight against the dark forces.
The plot could start with Harry having strange dreams about Shinigami. As he investigates, he discovers that there is a hidden portal in Hogwarts that leads to a realm where Shinigami exist. He and his friends enter this realm, and they find out that there is a dark force in the wizarding world that is related to the Shinigami in some way. They then have to work with some friendly Shinigami to stop this dark force. Along the way, they learn about new forms of magic and the true nature of death from the Shinigami.
The plot could start with a spell gone wrong in Hogwarts that somehow transports a group of Transformers to the wizarding world. The wizards are initially scared but soon realize that they need to cooperate with the Transformers to deal with a new dark force that is attracted to the combination of magic and technology. The story could involve battles, magic - tech hybrids, and new friendships between the two groups.