The characters in both universes have distinct personalities. To blend the styles, you could have a character from Harry Potter develop a trait that is more common in Mochizuki's characters, like a more introspective and philosophical side. And vice versa, a character from Mochizuki's world could learn the sense of camaraderie that is so strong among the Hogwarts students.
One idea could be a crossover where the magical world of Harry Potter intersects with the unique universe of Jiro Mochizuki's works. Maybe Harry and his friends discover a portal that leads to Mochizuki's world and they have to adapt to new rules and powers there.
One way to blend them is through the plot. Suppose there is a powerful dark force that is a threat to all three worlds. The heroes from Harry Potter, the Nephalem, and the characters from DxD would have to come together to fight it. The Nephalem could use their skills from their own world, like demon - slaying abilities, in the magical world of Harry Potter. In DxD, they could use the magical knowledge from Harry Potter to outwit their enemies. And the different cultures and power hierarchies from each world would create a rich tapestry for the fanfiction.
One way is through the use of magical artifacts. In a 'Harry Potter science fanfiction', a time - turner could be studied from a scientific point of view. Scientists (wizard scientists) might try to figure out the physics behind traveling through time, looking at concepts like relativity and how magic circumvents them.
Authors need to first understand the core elements of both universes. For Harry Potter, it's the magic, Hogwarts, and the fight against Voldemort. In KHR, it's the Mafia families and the growth of Tsuna. Then, they can start by introducing characters from one universe into the other in a believable way. For instance, if Harry were to enter the KHR world, how would he adapt to the different rules and powers there?
The themes blend by pitting the light - hearted and somewhat familiar magic of Harry Potter against the mind - bending, incomprehensible horror of Cthulhu. For instance, the concept of good vs. evil in Harry Potter takes on a new dimension when faced with the amorality of Cthulhu - like beings. The wizards' sense of order and their magical laws are challenged when these chaotic and eldritch forces enter their world.
Well, in some fanfics, the honor code of the Yautja might blend with the Gryffindor's sense of bravery. A Yautja might respect a Gryffindor who shows great courage in the face of danger, just as they respect worthy opponents in their own culture.
The worlds might blend through the use of shared magical elements. For instance, both have magical creatures. The gargoyles' ability to turn to stone during the day and come alive at night could be related to some form of Transfiguration magic in the Harry Potter world.
In a fanfiction, the blending could occur in a more dangerous situation. Let's say there is a rift between the two worlds and the magic is getting mixed up in a chaotic way. The characters have to figure out how to control this new, combined magic. They might find that some spells from Harry Potter work differently when combined with the energy of the devils in Highschool DxD. For example, the Killing Curse might be affected by the demonic energy and instead of just killing, it could turn a target into a half - devil. This would force the characters to understand both magic systems deeply and find a way to separate or control the combined magic before it causes more harm.
One way is through the use of magic and advanced technology. For example, Guinan's knowledge of time - travel technology could be combined with the time - turning magic in the Harry Potter universe. This would create really interesting plotlines.
It can blend the two worlds by having the droids use their technological features in the wizarding world. For example, a droid's built - in map system could be used to navigate the complex corridors of Hogwarts. The droids could also introduce new concepts like energy sources that wizards might be able to adapt for their magic.