One idea could be that Percy struggles to control his new vampire instincts while also trying to hide his identity from his friends. Another could be that he discovers a hidden vampire society within his town and has to navigate their politics.
A possible plot could be that Percy is born a vampire but his family doesn't know. As he grows up, he starts to show signs of his vampirism like a dislike for sunlight and a craving for blood. He then has to figure out how to get blood without harming anyone. He might meet other vampires who offer different ways of living as a vampire, some more ethical than others. This could lead to a journey of self - discovery for Percy as he decides what kind of vampire he wants to be.
Well, perhaps Percy's vampire nature gives him some unique powers that he has to learn to use. He might use these powers to protect his loved ones from other supernatural threats. Also, he could have a conflict with an older vampire who wants to turn him into a more 'traditional' vampire.
Maybe Percy has to deal with other gods being jealous or threatened by his sudden ascension to godhood. He could face challenges from them to prove he's worthy of being a god. This would involve epic battles and political intrigue in the world of the gods.
Another interesting plot could involve a vampire coven that has some sort of connection to the Greek gods. Maybe they want Percy for his unique demigod blood, which they believe has special powers. So Percy has to fight against this coven to keep his blood and his friends safe. This would bring in elements of both the Percy Jackson world and vampire lore. And it could lead to some really cool battles and alliances.
Maybe a demigod gets turned into a vampire in a strange magical accident. This would create a whole new set of challenges as they try to balance their demigod powers with their new vampire nature. They might have to deal with the vampire politics in Mystic Falls while also trying to stay true to their Olympian heritage. It could lead to some really interesting character development and power struggles.
Well, perhaps in this fanfiction, Percy is born a werewolf due to some dark magic that was cast on his family long ago. His journey could be about finding a way to break the curse while still being a part of the demigod world. He could meet other werewolves who either want to help him or are against him because of his demigod side. And he might have to face challenges like being hunted by hunters who don't distinguish between 'good' and 'bad' werewolves.
Another plot could be Percy being sent on a quest by Poseidon to retrieve a powerful and ancient artifact that was stolen from the sea god's domain. Along the way, he meets other demigods who have their own motives regarding the artifact, and he has to figure out who to trust.
Perhaps Percy could be in a constant battle within himself. On one side, his normal demigod self with loyalty to his friends and Camp Half - Blood, and on the other, the chaotic side of him that wants to disrupt and change things. He might meet other 'children of chaos' and they could either be his allies or enemies. And his journey could involve learning about the true nature of chaos and how to use his powers for good, like protecting the world from a greater chaos that threatens to engulf it all.
One possible interesting plot line could be Stiles struggling to hide his true vampire nature from his friends. For example, he might have to constantly come up with excuses for his super strength or aversion to sunlight. Another could be his discovery of an ancient vampire prophecy that he is a part of, leading him on a journey to fulfill it or avoid it. Maybe he also has to deal with vampire hunters who are specifically targeting him because of his unique 'born vampire' status.
Perhaps there could be a plot where Bella discovers she has a unique vampire ability that sets her apart from others. This could lead to her being both feared and revered within the vampire world. And she has to figure out how to use this ability for good or face the consequences of misusing it.
One idea could be that as a girl, Ichigo discovers new friendships in a different way. Maybe she forms a closer bond with female characters who previously saw her as a rival when she was male. Another could be that she has to deal with different expectations in the Soul Society as a female Shinigami, like different training regimens or social norms.
One idea could be the dragon having to hide its true identity in Hyrule. Maybe it discovers a long - lost power within the land that only it can unlock.