Well, it could be that the Night Fury is a result of some dark magic experiment gone wrong in the past in the Harry Potter world. It has now become a wild and free - flying creature that is feared by many wizards. However, Harry, being the brave one, tries to understand it and tame it, much like he did with Buckbeak. He realizes that the Night Fury is not as bad as people think and uses it to fight against Voldemort's forces.
One possible plot could be that Harry discovers a hidden Night Fury in the Forbidden Forest. He realizes that the dragon has some sort of connection to an ancient magic in the wizarding world. So, he tries to protect it from those who would want to use it for dark purposes. Along the way, he learns new spells and techniques from the Night Fury, which he then uses to fight against Voldemort's forces.
Well, Celebi is a Pokémon. In a Harry Potter fanfiction, it could be introduced as a very rare magical creature from a far - off land. Maybe it has time - traveling abilities like in the Pokémon world, and in the wizarding world, it could be used to correct historical mistakes or prevent dark wizards from rising in the past. For example, Dumbledore might discover Celebi and use its power to gain more knowledge about Voldemort's early days.
One idea could be that a Night Fury somehow gets transported into the Harry Potter universe. Maybe it crashes into the Forbidden Forest and Harry and his friends discover it. They could try to figure out how to communicate with it and use its abilities to their advantage against the dark forces in Hogwarts.
In a crossover, a female Harry Potter could be like a new recruit to the Avengers. She would start off being a bit of an outsider, but gradually earn their respect. Her magic would seem strange to the Avengers at first, but they would soon realize how useful it could be. She could help with things like getting into secure places that are protected by magic - like barriers in the Marvel universe that normal means can't penetrate. And she could also use her Patronus charm to inspire and protect the team during tough battles.
Well, Anubis could be introduced as an ancient and mysterious being from a different magical realm. Maybe he could be a part of some long - lost magical lore that Hermione stumbles upon in the library. For example, there could be a prophecy involving Anubis and a Hogwarts student.
Well, if we consider the Joker in a Harry Potter fanfiction, he might be an out - of - place figure. The wizarding world has its own set of rules and villains. But the Joker could be a new kind of threat. He could form an alliance with the Death Eaters, bringing his own brand of insanity to Voldemort's cause. Or he could be a lone wolf, just causing trouble for the sake of it, like messing with the Ministry of Magic's bureaucracy in really strange and unforeseen ways.
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.
A white tiger as a familiar in Harry Potter fanfiction could work in various ways. It could be from a different magical realm and somehow end up in the wizarding world. It would bring with it a new set of magical skills and instincts. For example, it might be able to sense dark magic from a greater distance than normal familiars. And when it comes to the Hogwarts environment, it could form an interesting dynamic with the school's mascots, like the Gryffindor lion, creating a contrast between wild and domesticated magic.
He could use his invisibility cloak for stealth missions with the Avengers.
Harry Hook often fits in as an alternate version or a new character added to the existing Harry Potter universe in fanfiction. He can bring new storylines and interactions.