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 crossover could start with a magical accident that links the three universes. For instance, a spell gone wrong in the Harry Potter world creates a portal that sucks in an Assassin and also connects to the Avengers' world. Harry Potter, being curious and brave, steps through the portal. Once in the combined world, they discover that there are forces at work that want to use the combined powers of magic, assassination skills, and superpowers for evil. So, they must form an uneasy alliance. The Avengers provide brute force and resources, the Assassin offers covert operations skills, and Harry Potter and his comrades contribute magic and knowledge of the arcane.
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.
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.
Well, Thor and Hagrid would probably get along great. They both have a love for big, powerful creatures. Thor could be interested in the magical beasts in the Forbidden Forest, and Hagrid would be in awe of Mjolnir. Then there's Iron Man and the Weasley twins. The twins would be fascinated by Stark's technology and would try to figure out how to incorporate some of it into their joke products. Stark, in turn, might be amused by their creativity and chaos - causing abilities.
A crossover could function through a portal or some sort of magical event that links the two worlds. For instance, the Tesseract in the Avengers universe could malfunction and open a portal to the Harry Potter world. Then, the Avengers find themselves in a world full of magic and wizards. They soon learn about the God of Death and realize that they need to work with the wizards like Harry, Hermione, and Ron to stop this ultimate force. The Avengers' powers would be a new and strange thing in the Harry Potter world, and the wizards' magic would be equally fascinating to the Avengers.
She would be a great addition. Her skills in combat and espionage would give her an edge in the wizarding world. She could easily adapt to the dangerous situations in both universes. For example, in Hogwarts, she could use her stealth to move around the castle undetected, much like she does in enemy bases in the Avengers world.
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 common enemy. For instance, a powerful crime syndicate that has ties to both the Avengers' world (like Hydra having some sort of deal with the syndicate) and John Wick's world. This would force them to collaborate.
One way it could work is by having Harry's magical abilities blend with the Avengers' powers. For instance, his ability to apparate could be used for quick getaways or surprise attacks. His Patronus could be used to fend off certain enemies that are affected by positive magic. Harry could also introduce the concept of magical creatures to the Avengers, like having a Hippogriff assist in a mission.
Perhaps in a fanfiction, Voldemort has made an alliance with a powerful villain from the Avengers' world and an ancient order of Assassins gone bad. Harry Potter, along with some of his Hogwarts friends, has to convince the Avengers to help them. They would then combine their different abilities. For example, Hermione could use her vast knowledge to figure out the weaknesses of their enemies, while Captain America could lead the group in battle strategy, and the Assassin could provide crucial intel on the secret workings of the enemy alliance.
Iron Man's technology could be combined with the magic of the wizarding world. For example, he could use his repulsor rays along with a magic amplification spell. Harry Potter's ability to use the Patronus charm could be used in tandem with Captain America's leadership skills to inspire the combined group in a battle.