Well, perhaps Harry could be transported to Soul Society. He might discover new magical abilities related to souls there. Maybe he'd interact with the Soul Reapers, learn their ways of handling souls and spirit energy, and use his own magic in unique ways within the context of Soul Society.
In a 'Harry Potter in Soul Society' fanfic, the plot could involve Harry arriving in Soul Society due to some magical mishap. He could end up in the Rukongai and have to survive there, making friends with some of the residents. As he explores, he realizes that his Hogwarts magic has different effects in this new world. He might get involved in the power struggles between the different Soul Reaper squads, using his quick thinking and magical skills to navigate the complex politics.
Obviously, Harry Potter would be one of the main characters. He'd bring his Hogwarts knowledge and magic skills. Also, Ichigo Kurosaki could be a main character. He could show Harry around Soul Society and they could learn from each other. Rukia Kuchiki might also be a main character as she has a deep understanding of Soul Society and could help Harry adapt.
Well, it could be about Harry Potter having to face a new kind of dark magic related to the demon's soul. Maybe he discovers a hidden artifact that is infused with the demon's soul and has to figure out how to destroy it without getting corrupted himself.
Well, it could be about Naruto being transported to the Soul Society and using his ninja skills there. Maybe he becomes a captain by showing his amazing combat abilities and unique jutsu. He might also interact with the existing Soul Reapers, teaching them some of his ninja ways while also learning about their powers.
One possible plot could be that Harry discovers he has the ability of a Soul Reaper. He then has to balance his life at Hogwarts with his new - found duties as a Soul Reaper. For example, he might have to deal with Hollows (evil spirits in Soul Reaper lore) that are lurking around the Forbidden Forest.
The fanfic could center around Harry's life outside of the main events of the books. For example, it could explore his summer holidays in more detail. He could be spending time in the Muggle world and using his magic in creative ways to solve normal, everyday problems. He might use magic to help his neighbor fix a broken fence without the neighbor realizing it was magic. This would add a new dimension to his character and show how he can adapt his magical abilities to different situations.
Well, perhaps Harry could be a weapon that has the power of his magic from the Potter world. He might be used by a Soul Eater character to fight against evil souls. Maybe his wand could transform into a more weapon - like form in the Soul Eater world.
Well, perhaps the plot could revolve around Harry being cursed in a way that prevents him from eating. His friends would then scramble to find a cure. They might visit different magical creatures or search through ancient spells in the library at Hogwarts. Hermione would be poring over books, Ron would be trying to keep Harry's spirits up with his jokes, all while they face the threat of Harry growing weaker without food.
One possible plot could be that Harry's muteness is a result of a dark curse. He then has to go on a journey to find a cure while still facing the challenges of the wizarding world, like fighting Voldemort. His friends would play a crucial role in helping him communicate and overcome the obstacles. For example, Hermione might develop a special device using magic to help Harry 'speak'.
Perhaps the plot involves a group of enslaved house - elves who decide to fight for their own rights rather than relying on the kindness of wizards. They could form an alliance with some of the human characters who oppose the slave - holding system. This would bring a new perspective to the story, as house - elves are often seen as subservient in the original series. It could also explore the complex relationships between different magical species and how they interact in a world full of inequality.
In this fanfic, as Harry is a cat, he could be seen as an omen or a special sign by some of the wizarding community. There could be a prophecy related to a cat that Harry now fulfills. He might befriend other magical cats in Hogsmeade or Hogwarts and together they uncover a plot by some dark wizards. His cat form could also give him unique abilities like seeing in the dark better or having a heightened sense of smell, which he uses to solve mysteries around the school. For instance, he could detect a strange potion being brewed in the dungeons just by the smell. And his friends would have to protect him from those who might see a cat as just an ordinary animal and not the important Harry Potter.