He could gain new magical abilities. For example, if he becomes a phoenix, he might be able to regenerate from ashes and have the power of flight in a more natural way than with a broomstick. This would change his role in the wizarding world significantly.
If Harry Potter were to become a centaur, he would likely have a very different perspective on magic. Centaurs have their own unique form of magic and knowledge. He might be shunned by some humans in the wizarding world, but gain respect among the centaur community. He could also use his new centaur form to explore the Forbidden Forest more freely and learn secrets that were previously unknown to him.
Suppose Harry becomes a hippogriff. His sense of pride and honor would be enhanced as hippogriffs are very proud creatures. He would need to be treated with respect, and he would also have to learn to interact with wizards and witches in a different way. His physical strength and ability to fly at high speeds would make him a formidable presence. He could use these new abilities to fight against dark wizards in a more unexpected way, perhaps swooping down from the sky and catching them off guard.
It could be a hippogriff. Hippogriffs are very majestic and intelligent magical creatures. In the Harry Potter series, Buckbeak was a hippogriff that Harry had an encounter with. It has the body of a horse and the wings and head of an eagle, which makes it both powerful and graceful.
In this 'Harry Potter goes to a creature school fanfiction', Harry Potter could have adventures with different magical creatures. He might get into trouble trying to handle some of the more dangerous ones, like a hippogriff that doesn't like him at first. He could also study the habits and habitats of various creatures, which would help him in his future battles against the dark forces, as knowledge of creatures can be really useful in the wizarding world.
Well, he might completely change the power structure in the wizarding world. He could rewrite the rules at Hogwarts, for example, making it more focused on combat training against the dark arts. He might also form a new alliance with the magical creatures, using their unique abilities to strengthen his rule.
Harry could also be a great Quidditch coach. He was an excellent Quidditch player himself. He would be able to train students in the skills of flying, teamwork, and strategy. He could pass on his love for the sport and perhaps even lead the school team to many victories. He would know how to spot potential in young players and help them develop their talents.
In a Harry Potter becomes a Pokemon fanfiction, Harry would likely bring his Gryffindor courage to the Pokemon world. He could be drawn to fire - type Pokemon like Charmander because of their similar fiery spirits. He might also use his invisibility cloak to his advantage while exploring the Pokemon habitats. Harry would probably try to understand the Pokemons' powers and abilities in the way he learned about magic at Hogwarts. He could also introduce the concept of houses to the Pokemon trainers he meets, creating a new kind of bond among them based on the values of Gryffindor, Hufflepuff, Ravenclaw and Slytherin.
If Harry Potter becomes a lantern fanfiction, there could be some really interesting crossovers. Maybe Harry could use Green Lantern's power ring in the wizarding world. He might use it to create magical constructs that combine with his spells, like a Patronus made of green energy. It would change the whole dynamic of his battles against Voldemort and his Death Eaters.
Well, he may face a lot of challenges at first. The other Gryffindors might view him with suspicion. However, as a Slytherin, Harry could start to explore the more complex magic that Slytherin students are often associated with. He might befriend some of the pure - blood families in Slytherin, which would give him a different perspective on the wizarding world's politics. And he could end up being a bridge between the two houses that are often at odds in the original story.
In such a fanfiction, Harry Potter as a Hollow might lose his sense of self and humanity. He could be driven by basic instincts rather than his usual moral compass. This would likely lead to a very different dynamic in the wizarding world as his friends would be faced with either trying to save him or having to oppose him.
In such a fanfiction, Harry might discover new powers related to being a monster. For example, he could have enhanced strength or senses. Maybe he would struggle to fit in with his old friends at Hogwarts as they might be afraid of his new form.