A really interesting concept would be a time - travel fanfiction. Black Widow travels back in time and ends up in the Marauders' era in the Harry Potter world. She befriends the Marauders and then later, when the events of the Avengers start to unfold, she brings some of her wizard friends to help out the Avengers. This could lead to some really unique interactions between characters from both universes.
In a fanfiction, the Avengers could be called to the Harry Potter universe to help fight Voldemort. Black Widow would be a key player in infiltrating Death Eater strongholds. She could teach the Order of the Phoenix some of her combat and espionage techniques. And perhaps Harry Potter could gain some inspiration from the Avengers' teamwork and use it against Voldemort.
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.
One idea could be a crossover where Hydra discovers the wizarding world in the Harry Potter universe and tries to use magic for their evil purposes. The Avengers somehow get wind of this and team up with Harry Potter and his friends to stop Hydra. Maybe they have to battle magical creatures that Hydra has corrupted with dark magic.
One idea could be that Harry Potter, as an Animagus, somehow gets transported to the Marvel universe. He could team up with the Avengers in his animal form, perhaps using his unique abilities to gather intelligence. For example, as a stag, he could move quickly and unnoticed in certain terrains, providing valuable information to the Avengers before a battle.
One idea could be a magical crossover where a spell goes wrong in the Harry Potter world and somehow links to the Avengers universe. A male character from either side could end up pregnant due to some strange magical - scientific mishap. For example, if Loki were to be affected by a Hogwarts spell that causes mpreg. It could lead to a lot of chaos as the Avengers try to figure out what happened while the Harry Potter characters deal with the consequences of their spell reaching another dimension.
One idea could be that the Deathly Hallows from Harry Potter have some sort of connection to the powers of the God of Death in the Avengers universe. Maybe the Elder Wand could be a weapon that has the power to control the souls like the Infinity Stones do in the Avengers. The Invisibility Cloak could be used by a character from the Avengers to sneak around in a world filled with magic, similar to how Harry used it at Hogwarts.
One idea could be a crossover where the characters from Harry Potter end up in the world of Black Butler. For example, Harry, Hermione and Ron could be mistaken for some kind of strange magical beings in the Victorian era of Black Butler. They might be brought to the Phantomhive manor and Sebastian has to figure out what to do with them. Maybe they are there to solve a mystery that involves both the magic of the Harry Potter world and the dark secrets in the Black Butler universe.
One idea could be that Black Widow is somehow transported into the Percy Jackson universe. Maybe she has to team up with Percy and his friends to defeat a common enemy. Since she has her combat skills and spy training, she could offer a different perspective on battle strategies compared to the demigods.
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.
One idea could be that Fluttershy somehow gets transported into the Harry Potter world. Maybe she arrives at Hogwarts as a new kind of magical creature that the students have never seen before. Her gentle nature might initially cause confusion among the more rowdy students, but she could gradually make friends with the likes of Luna Lovegood, who would be intrigued by her uniqueness.
Another concept could be that Harry Potter discovers he has a connection to Magneto's mutant powers. This might happen during a strange encounter with a dark magic object that unlocks a latent power within him. He then has to learn how to control these new abilities while also dealing with the threats in his own wizarding world, and he might seek out Magneto for guidance in a secret meeting across the boundaries of their two worlds.