The Chosen Undead would likely have a different understanding of magic compared to the wizards in Harry Potter. Their magic is often more about survival and combat. In the Harry Potter world, magic is also used for daily life and academics. This contrast could lead to interesting interactions. For instance, they might be baffled by the concept of Quidditch as it's a very different use of magic compared to what they're used to, but at the same time, they could introduce their own form of magical dueling that is more brutal and direct.
The Chosen Undead might be seen as an outcast at first. Since they are undead, they don't quite fit into the normal categories of wizards and witches. However, their combat skills and knowledge of different kinds of magic could earn them respect. For example, they could teach the students at Hogwarts some unique defensive spells that they've learned in their own world.
The Undead King can add an element of mystery and the macabre to the Harry Potter fanfiction universe. It could be a powerful and ancient being that exists on the fringes of the magical world. His powers might be based on necromancy or some other dark and forbidden magic.
In a fanfiction, the True Longinus might be a gift from a mysterious figure. This figure could be an ancient wizard who has been observing Harry. The True Longinus then becomes a tool for Harry to combat the growing darkness in the wizarding world. It could have unique magical properties that are different from the wands and magic we know in the Harry Potter series. For instance, it might have the ability to nullify dark magic spells completely, which would be a huge advantage for Harry.
It would be a very difficult fit. Since the wizarding world has its own norms like monogamous relationships in most cases (like the marriages we see in the series). If one were to introduce concubines, it would have to be in an alternate universe where the social and moral values are completely different. Maybe in a fanfic where the wizarding world is modeled after a more ancient and permissive society, but this would be a huge departure from the original feel of the Potterverse.
Mileena could be a foreign exchange student of sorts from a different magical realm. Her unique appearance and combat skills would make her stand out at Hogwarts. She might have trouble adjusting to the more academic and rule - based magic of the wizarding world compared to her own more instinctive and combat - focused magic.
He'd fit in by sharing his magic skills. His ability to perform spells without using hand gestures (unlike most Fairy Tail mages) would be a novelty. He could also bond with the other members over their love for magic.
He could fit in as a unique hero. His magic could be seen as a special power like the others in DC. For instance, he could use his Patronus charm to fight off fear - inducing villains.
Alucard could be a part of an ancient prophecy in the Harry Potter fanfiction. His arrival in the wizarding world could be foretold. He might be seen as a savior or a bringer of chaos. His powers could be seen as a necessary evil to defeat a greater threat, like a new and more powerful form of Voldemort. He would bring his own brand of violence and mystery to the relatively more structured world of Harry Potter, making for a very exciting and unpredictable story.
One way is through power sharing. If Harry's soulmate is an Avenger, they could combine their powers. For instance, if it's Iron Man, they could use magic to enhance the technology. Magic could be used to make the suit more powerful or give it new abilities like cloaking. The soulmate bond would make them work in harmony, understanding each other's strengths and weaknesses.
Hawkgirl could be a new kind of magical creature. Her ability to fly with her wings would be a unique form of magic in the wizarding world. She might be a protector of some hidden magical place in Hogwarts or the Forbidden Forest, like a guardian of an ancient magic that Voldemort is after.
The Chosen Undead could bring their unique skills, like the ability to wield powerful magic weapons, to the Harry Potter world. They might get involved in the fight against Voldemort, not because they care about the wizarding world's politics, but because they sense a similar kind of dark power in Voldemort that they've faced in their own world. Their presence could change the course of the battle in unexpected ways.