One possible plotline could be Percy's journey of grief and how he copes with the loss. He might become more withdrawn at first, then gradually find strength in his friends and his quest. Another could be his search for a way to bring his mom back, perhaps delving into dangerous magic or making deals with powerful beings. Maybe he also has to face the fact that his mom's death was part of a larger plan by the villains and he has to stop them from achieving their ultimate goal.
In this type of fanfiction, a plotline could be Percy going on a revenge spree against those he blames for his mom's death. He might abandon his usual moral code for a while as he is consumed by anger. Or, it could be about how the world around Percy changes after his mom's death. For example, his relationship with his father could become more complex as they both deal with the loss in different ways. Also, his friends might rally around him in unexpected ways to help him through this tough time.
One possible plotline could be that Percy sacrifices himself to save Artemis from a powerful and ancient enemy. Maybe there's a prophecy that foretells his death in order to protect her. Another could be that Percy is tricked into a situation where he has to give his life for Artemis to prevent a great disaster from befalling the world of the gods. And perhaps in his death, he becomes a symbol of heroism that changes the way the gods view demigods.
Another plotline could be that Percy's reincarnation is part of a grand plan. There could be a powerful being or a prophecy that dictates his reincarnation. When he is reborn, he is fated to either save or destroy this new world he is in. Along the way, he meets friends from his past life who have also been reincarnated, but they don't recognize each other at first, and they have to go through a series of adventures together to remember who they really are.
One possible plotline could be that Percy's sacrifice leads to a new era of peace in the fictional world. His death might inspire the other characters to come together and fight against the common enemies more fiercely. For example, his friends could form a stronger alliance to protect the world he gave his life for.
Percy might become more independent. Without his mom's presence, he has to rely more on himself. He could start to take on more leadership roles within the demigod community. He also may develop a darker side to his personality, as the pain of his mom's death lingers. However, through his experiences and the support of his friends, he gradually learns to balance this new, harsher side with his original kind - hearted nature.
It might also be related to some sort of enemy or threat that specifically targets Percy and his relationship with Annabeth. The enemy could be so powerful that Percy realizes the only way to stop it from harming Annabeth is to sacrifice himself, like if the enemy feeds on his life force or his connection to Annabeth, and he chooses to end it by sacrificing his own life.
One possible plotline could be that Hermione travels back in time and becomes Harry's mother. This would involve a lot of time - travel magic and dealing with the consequences of changing the past.
Perhaps there is a plotline where the family is in danger, and both Tony and Percy have to combine their skills, Tony's scientific skills and Percy's magical abilities, to save the day. Or it could be about Percy going to live with Tony for a while and the culture shock that follows for both of them.
Maybe in the fanfiction, Naruto dies due to an incurable disease. Hinata is heartbroken. She starts to isolate herself at first. But then, remembering Naruto's unwavering spirit, she decides to get involved in the village's affairs more actively, especially in areas that Naruto was passionate about, like helping the orphans. And she might also try to find a way to bring back Naruto, perhaps through some ancient jutsu that she discovers.
One possible plotline could be that Kakashi is in a life - threatening situation during a battle against a powerful enemy. Naruto, seeing no other way to save him, uses a powerful and self - sacrificing jutsu that costs him his life. Another plotline might involve Naruto making a deal with some dark force to save Kakashi, but the price is his own life. Or perhaps in a mission gone wrong, Naruto takes a fatal blow meant for Kakashi.
Perhaps there could be a plotline where Percy's mom's wizard heritage is somehow linked to the world of the gods. This could lead to a discovery that the two realms of magic and godly powers are more intertwined than anyone thought. It could also bring about a conflict where the gods are either threatened by her wizard powers or try to use them for their own purposes.