Here's a plot. Harry turns into a dragon in the Forbidden Forest. At first, he's scared and confused. But then he realizes he can fly and explore parts of the forest that are unreachable as a human. He discovers a hidden magical artifact while in his dragon form. However, the Ministry of Magic hears about his transformation and wants to capture him, thinking he's a dangerous uncontrolled magical being. So, Harry has to evade capture while also trying to figure out how to turn back to his human form and clear his name.
One idea could be that as a dragon, Harry uses his new form to protect Hogwarts in a way he couldn't as a human. He could fly around the castle, breathing fire at any dark forces approaching. Another could be that his dragon form has unique magical abilities that are connected to his status as 'God Harry Potter', like being able to see into the future to prevent disasters.
Another plot could involve Natsu turning into a dragon in a completely different world. He has to adapt to this new world as a dragon and find a way to turn back. Along the way, he meets other dragons who teach him about his new form and its capabilities. This could lead to him having a greater understanding of his own powers when he finally returns to his normal self.
One idea could be a character who is initially a lowly servant in King's Landing. After a strange encounter with an ancient magic, they start to transform into a dragon. As they gradually turn, they gain new powers like the ability to breathe fire in small spurts. This causes chaos as they try to hide their transformation from the power - hungry lords and ladies of the court. They might also start to have dreams of flying over the Seven Kingdoms, which drives them to seek out the knowledge to fully complete their transformation.
Perhaps Danny turns into a dragon in a completely unexpected situation, like during a school event. His sudden transformation causes chaos and he has to quickly fly away to avoid being seen. In the process, he discovers that he can access new areas in his dragon form that were previously inaccessible to him as a normal half - ghost. He might find a hidden lair of an old enemy in the mountains while flying around in his dragon form, and then has to figure out how to defeat this enemy while still learning to control his new abilities.
In this fanfiction scenario, Harry becoming a werewolf could cause a rift in the wizarding community. Some might see him as a threat, while others, like the Weasley family, would stand by him. The story could focus on how he copes with this new identity. He could start to understand the werewolves' perspective more, and try to bridge the gap between the wizarding world and the werewolf community. He might also have to deal with the Ministry of Magic's attempts to control or eliminate him because of his new form.
One idea could be that Harry has to hide his new werewolf form from his friends at Hogwarts. He might struggle with the stigma associated with werewolves in the wizarding world, even though his friends know him well. Another could be that he discovers new magical abilities related to his werewolf form that help him in his fight against Voldemort. For example, enhanced senses that allow him to detect dark magic more easily.
Another interesting concept is Harry turning into a cat. He could then slink around Hogwarts unnoticed. He could spy on Snape or Malfoy, gathering information. His cat form would give him the ability to be stealthy and get into small spaces, like the hidden passages in the castle.
A possible plot is that Shirou as a girl gets caught up in a romantic sub - plot that was not possible before. For example, she might be the object of affection for a character who previously only saw her as a friend when she was male. Also, she could struggle with her identity, trying to hold on to the values she had as Shirou while adapting to her new physical form. This internal conflict could drive the story forward.
One idea could be Izaya, as a girl, using her charm to manipulate situations in a different way than before. She might be underestimated at first because of her new female form, but then shock everyone with her still - sharp intellect.
One plot idea could be Sirius, as a girl, having to navigate the wizarding world's gender - based expectations. For example, she might face different treatment in Hogwarts classes or from other students. Another could be her relationship with her friends, like how James and Remus react to this change. Maybe they initially struggle to adjust but then come to accept her fully. Also, her interactions with the female characters could be explored, such as forming new friendships or rivalries with girls in her year.