In this fanfiction, perhaps a powerful dark magic object turns Harry into a cat. As a cat, Harry's senses are heightened. He can smell danger better and hear things from far away. He might use these new abilities to uncover a plot by Voldemort's followers. For example, he overhears some Death Eaters planning to attack Hogwarts through a hidden entrance that only a cat could access. So he has to find a way to warn his friends while still in cat form and then figure out how to turn back to human to fight against them.
The plot could involve Harry's transformation as a part of a new Dark Arts experiment. As a rabbit, he has to adapt quickly to survive. He could find that some of his magic is still accessible in this form, but it's much more difficult to control. He might use his small size and speed to his advantage, evading the spells of those who are after him. His friends could enlist the help of Hagrid, who has more experience with magical creatures. Together, they try to track down the person responsible for the transformation and turn Harry back into his human form.
Maybe he turns into a cat because of a misfired spell from a young, inexperienced wizard. And while in cat form, he finds that he can communicate with the magical cats around Hogwarts in a way he couldn't as a human. They tell him secrets about the Forbidden Forest that could help in protecting the school.
Another plot could involve a sort of magical exchange. Orochimaru, being a master of some dark and strange jutsu in his own world, might offer to teach some of his knowledge to certain dark wizards in the Harry Potter world in exchange for something valuable. Harry Potter and his friends would have to prevent this knowledge transfer as it could tip the balance of power towards the dark side in their world. They might need to seek help from some of the more powerful and wise wizards like Dumbledore to outwit Orochimaru.
Another plot could involve Medusa being a part of a long - forgotten magical mystery that is somehow related to Harry's destiny. Harry, with his inquisitive nature, starts to research about her. As he gets closer to the truth, he has to face the danger of being turned to stone. But through his courage and the help of his friends, he manages to break the curse on Medusa, and in return, she helps him in his fight against Voldemort.
One possible plotline could be that Naruto is hit by a strange jutsu that causes him to transform into a full - grown man overnight. He then has to deal with the physical and mental changes while still trying to protect the village. His friends might be initially shocked but then support him as he figures out how to reverse the transformation.
One possible plotline could be that Sonic gets bitten by a vampire during one of his adventures in a spooky zone. He then starts to notice changes in himself, like a craving for blood and enhanced speed at night. His friends initially don't realize what's wrong and think he's just acting strange. As he turns more vampiric, he has to struggle with his new nature and not harm his friends.
One possible plotline could be that Zuko turns into a dragon to gain a new perspective on his firebending. As a dragon, he might discover ancient firebending techniques that were long forgotten. This could lead him to return to his human form with enhanced skills and a new understanding of his power. He could then use this knowledge to help his friends and bring peace to the Fire Nation.
One possible plotline could be that Trunks, after turning into a girl, has to adapt to a whole new set of social expectations. For example, he might struggle with how others now perceive and interact with him. Maybe he was used to being seen as a strong male fighter, but as a girl, people might treat him more delicately at first. Another aspect could be his relationships. His friends might be a bit shocked at first but then come to accept him. And he could also have new romantic interests or challenges in that area, as his new form might attract different types of people.
One possible plotline could be that Uncle Matthew Potter has some long - lost magical power or artifact that the main characters need to find. This could lead them on an adventure through different parts of the wizarding world. Another plot could involve him having a secret connection to a dark wizard from the past, and the story could be about uncovering this connection and dealing with its consequences.
Perhaps Harry discovers that he has basilisk blood in him. This could change his abilities and how others view him. He might face discrimination from some in the wizarding world who fear the basilisk's power, while others might see him as a potential weapon against Voldemort. The plot could revolve around Harry coming to terms with this new aspect of himself.