One idea could be that Percy turns into a dragon to fight a powerful sea monster. Since he has his connection to water, as a dragon he could breathe a sort of water - based fire or ice that freezes the water around the monster, trapping it.
One common theme is self - discovery. When Lucy turns into a dragon, she has to figure out who she is in this new form. Another theme is power and its consequences. With the power of a dragon, she has to learn to control it or face the negative outcomes.
In some fanfictions where Natsu turns into a dragon, it often gives him extraordinary power. He might have enhanced fire abilities, being able to breathe more intense flames. His physical strength could also increase exponentially, making him an even more formidable force in battles. Maybe he would also gain some dragon - like instincts, which could either help him in combat or cause some internal conflict as he tries to balance his human side with his new dragon self.
One possible plot could be that Issei discovers a hidden power within himself that gradually transforms him into a dragon. This new form gives him enhanced strength and abilities, but also attracts the attention of powerful enemies who want to either control him or eliminate him. He has to learn to master his dragon form while also protecting his friends and loved ones.
One possible plot could be that Astrid is cursed by an evil sorcerer. At first, she has no idea what's happening to her but slowly starts to notice physical changes like her skin getting scaly. As she turns more and more into a dragon, she has to struggle to maintain her relationships with her friends who are initially afraid of her new form. Eventually, she discovers a way to break the curse through an ancient magic ritual.
One possible plot could be that Eragon, while facing a powerful dark enemy, discovers a hidden ancient magic that causes him to transform into a dragon. As a dragon, he has to quickly adapt to his new form, learning to fly and use his newfound dragon abilities like enhanced strength and fire - breathing. He then uses these powers to defeat the enemy and protect his land.
One common theme is the struggle for identity. When Link turns into a dragon, he often has to grapple with losing his human self and what that means for his relationships and purpose. For example, in many stories, he might worry about how Zelda or his other friends will view him in his new form.
Perhaps in the fanfic, Naruto turns into a dragon as a result of a forbidden jutsu gone wrong. In his dragon form, he discovers a connection to an ancient dragon spirit that guides him. This spirit could teach him new abilities and knowledge. His friends, like Sasuke and Sakura, would then have to help him figure out how to reverse the transformation or at least learn to live with it. They could go on a journey to find a way to control the dragon within Naruto, facing various challenges along the way, such as other ninja clans who see Naruto's new form as a threat.
In Naruto fanfiction, the concept of turning guys into girls can be a really interesting and creative idea. It might be used to explore different aspects of the characters. For example, it could show how a male character would adapt to the challenges and experiences that female characters typically face in the Naruto world. It could also be a way to add a new dynamic to relationships between characters.
I haven't heard of a fanfiction where Kyuubi turns Naruto into a futa. Naruto fanfiction often focuses on things like character development, new adventures, or different power - up scenarios. This concept is rather unorthodox and might not be as common as more mainstream fanfiction ideas. It's possible that it exists in some corner of the vast fanfiction universe, but it would take some serious searching to find it.