One possible plot could be that Harry is under a powerful dark spell. The transformation into a dragon might be a part of a scheme by Voldemort or his followers to either discredit Harry or use his new form for some evil purpose. Maybe Harry has to learn to control his dragon form in order to fight back and protect Hogwarts and his friends.
One possibility is that Harry Potter becomes a hybrid of a wizard and a centaur. He would inherit the centaur's knowledge of the stars and nature. His lower body being that of a centaur would make it difficult for him to move around Hogwarts in the normal way. He would have to find new ways to get to his classes. And he might be shunned by some students who are afraid of his new and strange appearance, but he could also form new alliances with those who are more open - minded and interested in his unique situation.
In a 'Harry Potter turns to the Dark Side' fanfiction, the plot might involve Harry being influenced by a powerful dark object. For example, a cursed artifact that makes him see the Dark Arts in a different light. He could start to believe that the ends justify the means and that the Dark Side has been misrepresented all along. As he delves deeper into the dark side, he begins to turn against his old friends, thinking they are too naive and weak to truly change the wizarding world.
In this fanfiction, it could be that a magical accident occurs during a potion - making class. Harry accidentally drinks a wrong potion which transforms him into a baby. Dumbledore then decides to keep Harry at Hogwarts, with the teachers taking turns to look after him. Meanwhile, Neville could be the one who discovers a clue about how to reverse the transformation by observing some strange behavior of magical plants in the school's greenhouse.
In the fanfiction, it could be that there is some ancient magic tied to Slifer that has a connection to Harry Potter. Maybe a hidden prophecy or a long - forgotten magical artifact causes the transformation. As Slifer, Harry might find himself in a situation where he has to communicate with other magical creatures in a different way. He could also face discrimination from some wizards who fear his new form, but also gain allies who recognize the potential of his new abilities.
In the fanfiction, Harry might be cursed to transform into a dragon periodically. He then has to figure out how to control this new form. He could meet other magical creatures that are either scared of him or intrigued by his dragon form. He might also find that his dragon form gives him unique powers that he didn't have as a human wizard, like the ability to fly faster or breathe a more powerful type of fire.
Well, perhaps in this fanfiction, Harry Potter discovers he has some sort of divine connection to the dragon god. Maybe he gains the ability to communicate with dragons and uses their powers to fight against Voldemort. He could also be on a quest to understand the nature of this newfound connection and how it relates to the wizarding world.
In such a fanfiction, perhaps Harry discovers a dragon in the Forbidden Forest. He realizes the dragon is being hunted by dark wizards. Harry, using his skills and magic, decides to protect the dragon. They form a bond as Harry spends time understanding the dragon's needs and communicating with it through magic. Eventually, the dragon becomes his ally in fighting against the dark forces in Hogwarts.
One possible plot could be that Harry has to keep his true dragon form a secret while at Hogwarts. He might face challenges like controlling his dragon instincts during classes or Quidditch games. For example, his natural inclination to hoard things could get him into trouble in the common room when he tries to stash away everyone's shiny objects.
Well, perhaps the plot could involve Harry finding a dragon egg and raising the dragon as his familiar. This would bring a whole new set of adventures as he has to hide the dragon from the Hogwarts faculty who might not be too keen on a dragon on campus. He could also use the dragon's special abilities to his advantage in his battles against Voldemort.
Well, perhaps Harry could discover he has some dragon - like abilities. Maybe he can breathe a bit of fire when he's extremely angry. It would make him even more of an outcast at first, but then these abilities could help him in battles against Voldemort.