Well, perhaps he could use his new dragon form to fight against Voldemort in a very unexpected way. He might be able to breathe fire on the Death Eaters and protect Hogwarts more effectively.
Well, it could start with Harry being cursed by a dark spell during one of his adventures near the Black Lake. Suddenly, he starts to feel strange sensations and discovers his legs are fusing together and turning into a merman's tail. He has to adapt to this new form. Maybe he discovers he can communicate with the merfolk in a different way now and they have some secrets about the wizarding world that only he can learn in this form.
In this type of fanfiction, an interesting plot twist might be that Harry, as a girl, is underestimated by many of the male characters at first. But then she uses this to her advantage. She could pretend to be meek and mild while secretly gathering information and planning strategies. Also, her transformation could lead to a change in the way Hogwarts is run. Maybe she starts a movement for more equality between male and female students, which causes a stir among the old - fashioned teachers.
In this fanfiction, it might be that a new and mysterious potion was accidentally spilled on Harry. As a child, he'd be super cute and all his friends would be really confused. Hermione would probably immediately start researching ways to reverse the spell. Ron, on the other hand, would be a bit more clumsy in dealing with this new young Harry. And Harry himself might have a lot of fun exploring Hogwarts in a new way, with the perspective of a child but the knowledge of an older wizard.
Dumbledore, if he were still alive in the fanfiction, would likely have some wise words and advice for Harry. He might have some secret knowledge about dragon transformations that could be crucial in helping Harry. Also, Neville could come in handy as he has proven to be brave and resourceful, and he might be able to find some rare plants that could be of use to Harry in his new form.
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.
The plot could involve Harry discovering a prophecy that links his destiny to slaying a powerful dragon. So, he embarks on a journey to find and defeat it. Along the way, he might have to learn new spells or combat techniques specific to dragon slaying.
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.
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.
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 dragon, being a powerful and wise creature, sees Harry Potter's potential and decides to raise him in a hidden lair. It could teach Harry unique magical skills that are different from what he'd learn at Hogwarts, like how to communicate with magical beasts on a deeper level.