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 this fanfiction, it could start with a strange experiment in the Potions class. Harry adds the wrong ingredient, which creates a magical mist that spreads through the classroom and turns everyone into Smurfs. The transformed students find that they have some of the characteristics of Smurfs, like being able to communicate with the forest creatures in a new way. Harry realizes that he has to use his knowledge of both wizardry and the newfound Smurf - like abilities to reverse the spell. He might encounter a Smurf - like version of Dumbledore who gives him some cryptic advice on how to break the spell.
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 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 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.
Ron would probably be both shocked and a bit awestruck at first. He might make some typical Ron - like jokes to ease the tension, like 'Blimey, Harry, you're a dragon! At least you'll be good for scaring off Snape now!'. But deep down, he'd be worried and determined to help Harry get back to normal.
One idea could be that Harry discovers a dark secret about his family that leads him down a dark path. For example, he might find out that his ancestors were involved in some really evil magic and he starts to explore that side of magic. Another could be that the influence of Voldemort somehow remains and corrupts Harry's mind over time. Maybe the Horcrux inside him wasn't completely removed and it starts to change his way of thinking.
The plot could start with Harry suddenly waking up as Luna. He would be confused and disoriented at first. Then, he would have to attend classes as Luna, which would be a challenge as he has to mimic her behavior and knowledge. Along the way, he might discover some secrets that Luna was aware of but he wasn't before. He could also use his own skills and knowledge, like his expertise in Defense Against the Dark Arts, to solve problems in Luna's form, all while trying to find a way to turn back.
A plot twist could be that Dumbledore faked his own death to test Harry's loyalty. But when Harry starts to turn on him, Dumbledore realizes his plan backfired. This would create a lot of tension and conflict between them, and also force Harry to re - evaluate his entire relationship with Dumbledore.
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.