In a fanfiction, a group of Animagus in the Harry Potter universe accidentally get transported to a world where Godzilla exists. They then have to use their Animagus abilities to survive in this strange new world filled with giant monsters. They could learn from Godzilla's power and incorporate it into their own magical abilities when they eventually find a way back home.
Perhaps a dark wizard uses magic to create an Animagus - Godzilla hybrid as a weapon. The Harry Potter gang has to find a way to stop this abomination. They might use their own Animagus forms to combat it. For example, Harry could turn into his stag Animagus and use its speed and agility to distract the Godzilla - Animagus while Hermione figures out a spell to reverse the dark magic.
One idea could be a character in the Harry Potter universe who is secretly an Animagus but gets cursed and starts to show Wendigo - like traits. They struggle to control their newfound savage nature while also hiding it from their friends at Hogwarts.
A fanfic could involve a character from Twilight who discovers they have the potential to be a wolf animagus in the Harry Potter sense. They then have to travel to the wizarding world to learn how to control this new ability. This would bring in elements of both worlds, like the magic of Hogwarts and the vampire - werewolf - human dynamic of Twilight.
One idea could be that Harry Potter, as an Animagus, somehow gets transported to the Marvel universe. He could team up with the Avengers in his animal form, perhaps using his unique abilities to gather intelligence. For example, as a stag, he could move quickly and unnoticed in certain terrains, providing valuable information to the Avengers before a battle.
In some fanfiction, Godzilla could be a guardian of a hidden magical realm within the Harry Potter universe. This realm contains powerful magic that could be used for good or evil. Voldemort tries to gain access to this realm to get more power, but Godzilla stands in his way. Harry and his friends then have to decide whether to help Godzilla protect the realm or try to find a way to use the power within for their own fight against Voldemort.
Another possibility is that Godzilla somehow ends up in the Naruto world. The shinobi might initially see it as a great threat, but Naruto could try to communicate with it using his ability to sense emotions. Maybe Godzilla is there because it's being chased by an even greater enemy from its own world, and Naruto and his friends decide to help it.
One idea could be that Harry discovers he has a strange connection to the Pokémon world. As an Animagus, he could transform into a form that has some similarities to Mewtwo, like having purple eyes or telepathic abilities. Maybe he gets transported to a world where Pokémon exist and has to use his skills as an Animagus and wizard to survive.
Perhaps a story about an adult wizard who was cursed and turned into a unicorn animagus against his will. He then has to navigate the wizarding world in this new form, facing prejudice from some who don't understand his condition. Along the way, he meets Harry Potter and his friends, who help him find a way to break the curse.
An Animagus might be initially terrified of Godzilla. If it's a small Animagus form like a mouse, it would be dwarfed by Godzilla's size. However, if it's a large Animagus like a bear, it might still be no match but could try to intimidate. For example, a wizard in his bear Animagus form could stand in front of Godzilla to protect his friends, even though he knows he can't really defeat it.
One idea could be that Fluttershy somehow gets transported into the Harry Potter world. Maybe she arrives at Hogwarts as a new kind of magical creature that the students have never seen before. Her gentle nature might initially cause confusion among the more rowdy students, but she could gradually make friends with the likes of Luna Lovegood, who would be intrigued by her uniqueness.
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.