How can a black doggy in a tub fit into the 'Harry Potter' universe in a fanfiction?

Well, perhaps the black doggy could be Sirius Black in his Animagus form. Maybe he's hiding in the tub because he's on the run from the Ministry or some Death Eaters. And a young wizard or witch discovers him there, and at first is scared but then realizes who he is and decides to help him.

Cassandra Black can fit in many ways. She could be a key figure in uncovering forgotten magic or prophecies. For example, she might have knowledge of an ancient spell that could help Harry defeat Voldemort.

Another way is that the Black Shuck is a manifestation of an ancient magic that has been long forgotten. A group of students in the fanfiction could discover some old texts about it and decide to investigate. As they dig deeper, they find out that this Black Shuck is connected to a much larger mystery that could change the way they view the history and magic of the wizarding world.

He might be a character who was involved in the politics of the wizarding world. Fanfiction could explore how his influence, either positive or negative, affected the balance of power. And this could in turn impact the events that Harry has to face. For example, if Arcturus Black had made certain alliances, it could change the dynamics of the war against Voldemort.

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.