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.
To fit concubines into a Harry Potter fanfiction universe, one would need to re - imagine the social fabric of the wizarding world. For example, if the story was set in a time when wizarding society was more influenced by a culture that practiced concubinage. But this would also mean changing many other aspects like family structures, inheritance laws, and social hierarchies. It could also lead to a lot of conflict with the existing characters' values, as most of the main characters in Harry Potter uphold values like loyalty and equality in relationships.
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.
In fanfiction, umbramancy can fit in by being a long - lost or forgotten magic. It could be something that was once used by ancient wizards but lost over time. For example, it might be rediscovered in the Forbidden Forest by a curious student.
Shukaku could be a powerful magical beast that has been sleeping deep within the grounds of Hogwarts for centuries. Only a very powerful spell or a unique combination of spells could awaken it. Once awakened, it could either be a great ally or a terrifying foe depending on how the wizarding community treats it.
Supergirl could be a foreign exchange student of sorts in the Harry Potter universe. She could be there to study magic while also sharing her own Kryptonian knowledge. Her superpowers would make her stand out, but she would also have to learn the rules of the wizarding world, like not using her powers when it's against the wizarding code.
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.
Illumi can fit in as an outsider with unique skills. His combat and stealth abilities are not like anything in the wizarding world, so he could be a wildcard. For instance, in some fanfictions, he might be seen as a new kind of threat or an unexpected ally to the Order of the Phoenix.
Freerunning can fit into the Harry Potter universe by adding a new dimension to movement. For example, it can be used for escaping from dark wizards in the narrow alleys of Diagon Alley or Hogsmeade. The characters can use their parkour skills to quickly jump over obstacles and get to safety.
She could be a powerful ally. Since Bella has enhanced physical abilities in her own story, in the Harry Potter universe, she could use those to help in battles. For instance, she could be part of the Order of the Phoenix and use her speed and strength to fight against Voldemort's forces.
Mithril might be incorporated into the fanfiction as a material that has a special connection to certain magical creatures. Maybe unicorns are attracted to Mithril, and a plot could revolve around a group of students trying to use Mithril to lure a unicorn to safety from a poacher. Along the way, they learn more about the magical properties of Mithril and how it interacts with the magic of the unicorn, which in turn could have implications for other areas of magic within the 'Harry Potter' universe.
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.