I've heard of 'Potter and the Hitman's Adventure'. In this fanfic, the author takes elements from both series and creates a new plot. For example, Harry could end up learning some of Reborn's combat techniques while still using his wizardry. It's a really fresh take on cross - over fanfictions.
There's also a fanfic where the Death Eaters somehow cross paths with the Mafia in KHR. It's called 'Dark Alliances'. The story shows how the two different types of villains from each series could either clash or cooperate. It makes for a very exciting read as it keeps you guessing about what will happen next.
There are often fanfictions that blend different universes like this. In some, Green Arrow's modern - day gadgets and skills might seem out of place in the magical world of Harry Potter at first. But then, the author might find a way to make it work, like his arrows being enchanted or having special properties that can counter dark magic. It's really all about the creativity of the writer.
One great fanfic could be 'The Wizard of Camelot'. In this story, Harry Potter is magically transported to the time of King Arthur. The author cleverly blends the magic systems of both universes. Harry uses his wand in a world where Merlin's magic also exists. It's really interesting to see how he interacts with the knights of the round table.
There are many fanfictions out there that blend these two universes. Some might focus on a scenario where the technology from Captain America's world is introduced to the wizarding world. It could lead to interesting developments like using Stark - like technology to enhance magical devices or creating new ways to defend Hogwarts. Another aspect could be the moral and ethical differences between the two worlds. Captain America's strong moral code might be put to the test in the morally gray areas of the wizarding world, like dealing with dark magic and its users.
I've read one where a young wizard from the Harry Potter world gets transported to the Star Wars galaxy and discovers technomancy there. It was really cool how he used his knowledge of magic to understand and sometimes modify the technomancy devices. The story had some great battles where his spells interacted with the energy - based weapons of the Star Wars universe.
You can start by looking on popular fanfiction websites such as Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, and you can easily search for Harry Potter KHR crossovers using the search bar. Just type in relevant keywords like 'Harry Potter KHR' and browse through the results.
There's also 'A Crossover of Worlds'. This fanfiction focuses on the clash of cultures between the two universes. It has interesting plot twists where the Death Eaters get involved with the Vongola famiglia. It's full of action and the author does a great job of making the two different magic systems work together.
One interesting Harry Potter and Nazgul fanfiction could be one where the Nazgul somehow cross into the Harry Potter universe. Maybe they are drawn there by the dark magic that Voldemort wields. A great place to start looking for such fanfics is on fanfiction.net. There are many works that blend different fantasy elements together in creative ways.
Well, 'Metamorphmagus Encounters' is also worth checking out. In this fanfic, the Metamorphmagus element is used to explore different magical abilities and how they interact with the wizarding world. It dives deep into the lore of Metamorphmagi and their place in the Harry Potter universe. There are some really cool scenes where the Metamorphmagus uses their powers in unexpected ways.
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.
One idea could be that Hawkgirl somehow gets transported to the Harry Potter universe. Maybe she arrives during a Quidditch match and her flying skills astound everyone. She could then join forces with Harry and his friends to fight against the dark forces in Hogwarts, using her Nth metal mace and flight abilities in unique ways.