Well, it could start with the Hulk being magically summoned to Hogwarts. He'd be confused by all the magic around him. The Hogwarts professors would be both fascinated and worried about this strange, incredibly strong being. They'd try to communicate with him, but the Hulk might just roar in response. Harry and his friends could be the ones to eventually make a connection with the Hulk, maybe by showing him kindness instead of fear.
In a more complex scenario, there could be a link between the magic in the Harry Potter universe and the science that created the Hulk. Some sort of ancient magic in the wizarding world has a parallel in the gamma radiation that created the Hulk. A villain discovers this and tries to use it to create an army of super - powered creatures. Harry Potter and the Hulk then have to work together to stop this villain. They would each bring their own skills and powers to the fight, with Harry using his spells and the Hulk using his raw strength.
In a fanfic, it could be that the magic in the Harry Potter world has an unexpected effect on Hulk. For instance, the magic might make Hulk's transformations more controllable, or it could give him the ability to communicate better. And in return, Hulk's super strength and durability could be used to protect Hogwarts from various threats, like a giant dragon attack or an invasion of dark creatures. This way, both elements of the two universes blend well and create an exciting story.
Well, it could work by integrating the magic system of Harry Potter with the power of Susanoo. For instance, the wizards in the Harry Potter world could learn to harness Susanoo's power through their magic training. They might use spells to activate it, like 'Accio Susanoo' to summon it.
One idea could be that the Hulk somehow gets transported to the wizarding world. Maybe a magical experiment gone wrong by a Hogwarts student creates a portal. The Hulk's brute strength would be a shock to the wizards and witches. He could end up causing chaos in Hogwarts, knocking down walls and scaring students. Wizards would try to use spells on him, but his strength might make him resistant at first.
Well, it could work by having a common threat. Say a powerful sorcerer from the Harry Potter world who has learned to harness some sort of high - tech power similar to what the Avengers deal with, and also has a network of assassins at his command. The heroes from each world would then come together. Harry Potter and his friends would bring magic, the Avengers their superpowers and technology, and the Assassin their unique combat and stealth abilities to combat this threat.
A possible way for a crossover to work is through a shared villain. Let's say a powerful dark wizard somehow gets hold of some Stark Industries tech and combines it with dark magic. This new threat catches the attention of both Spider - Man and Harry Potter. Spider - Man would bring his scientific knowledge and superhero skills, while Harry and his friends would contribute their knowledge of magic. They would have to learn to work together, with Spider - Man having to adapt to the strange rules of the wizarding world, like not being able to use his phone or other Muggle technology in Hogwarts.
It could work by having a magical event that blurs the boundaries between their two worlds. For example, a powerful spell gone wrong could open a portal between the wizarding world and the world of the vampires. Elijah, being curious and always looking for new things, would be drawn to this new world. He and Harry would then meet and their different skills and personalities would create interesting storylines.
It could also work through a shared threat. Imagine a villain who has both magical and scientific powers is threatening both the wizarding world and Central City. Harry Potter and Barry Allen are brought together by their need to stop this villain. Harry uses his spells and knowledge of the wizarding world, while Barry uses his super speed and scientific know - how. They have to learn to trust each other despite their different backgrounds and ways of doing things. For instance, Barry might be initially skeptical of magic, but as he sees it in action, he starts to respect it more.
A really interesting way would be if there was a prophecy in both worlds about an immortal being who would bring balance. A character from Harry Potter, like Dumbledore who has some form of extended life through his powerful magic, and a character from LOTR, say Gandalf who is a sort of immortal spirit, could be the key players. They could discover that they are both part of this prophecy and have to work together. Their combined wisdom and power would be used to fight against the common threats in both worlds, such as the dark wizards in Harry Potter and the forces of Sauron in LOTR. This would involve a lot of world - building and character development as they bridge the gap between their two very different universes.
Well, it could work by having a magical portal connect their two worlds. Vincent could step through it into the wizarding world. Then he would have to adapt to the rules of magic there. He might be confused by things like wands and spells at first, but his combat experience would be useful. For example, when facing a horde of Dementors, he could use his fast reflexes to fend them off while Harry uses his Patronus charm.
One way it could work is by having a threat that requires both the Hulk's strength and the Justice League's diverse powers. For instance, an alien invasion that has technology to nullify the League's powers partially. The Hulk's physical strength would be unaffected, so he could be the key to victory.