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 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.
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.
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.
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.
One possible plot could be that Sirius accidentally casts a spell that turns Harry into a baby. He then has to take care of Harry in this new form while trying to find a way to reverse the spell. He might face challenges like hiding Harry from the wrong people, and also dealing with Harry's baby - like needs such as feeding and changing diapers.
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.
In a 'Harry turns into a vampire' fanfiction, the story could start with Harry being cursed by a powerful and ancient spell that turns him into a vampire. His friends at Hogwarts are initially scared and unsure how to deal with him. However, Harry is determined to find a way to reverse the transformation. Along the way, he discovers that there are some positive aspects to being a vampire, such as heightened senses that help him in his detective - like investigations into the origin of the curse.
Maybe Harry is exploring an ancient and mysterious part of the Forbidden Forest. There, an old magic causes his wand to turn into a sword. The sword - wand seems to have a mind of its own and leads Harry to a hidden chamber. Inside, he finds a prophecy that can only be unlocked by the sword - wand. As he tries to figure out how to use it for this purpose, he has to fight off various magical creatures that are guarding the chamber. Eventually, he unlocks the prophecy, which reveals a great danger coming to the wizarding world and he sets off on a new quest with his sword - wand to prevent it.
If Harry Potter turns into a heartless in fanfiction, it would be a huge change from his usual self. His relationships would be in shambles. Hermione and Ron would be shocked and try to bring him back to his old self. He could ally with the Dark forces not because he believes in their cause but just for power. He'd disregard the feelings of others, like the way he cared about the younger students in Hogwarts before. This would create a very dark and different version of the Harry Potter world we know.
One possible plot could be that Harry's insanity is caused by a cursed object. His friends then have to find a way to break the curse while also dealing with his erratic behavior. For example, Harry might accidentally reveal secrets to the wrong people because of his insanity.