In the fanfiction, Percy's view of life and mortality would be completely different. He might not fear death as much as others do, and this could lead to some really interesting character development. For instance, he could be more reckless in some situations because he has a different understanding of what it means to die, or he could be more empathetic towards those who are about to die.
In this fanfiction, Percy's role in the prophecy could change. Instead of being the hero for the Olympians, he could be the one to bring a new order with his Titan powers. His connection to Poseidon would be strained or even broken. He would have to find new allies, perhaps among the other demigods who are also outcasts because of their different heritages. And his battles would be against different foes, not just the Titans' old enemies but also those who oppose the idea of a demigod with Titan blood.
The fanfiction could also explore Percy's identity crisis. He's known himself as the son of Poseidon for so long, but now being the son of Gaea and Tartarus would shatter that identity. He would have to re - evaluate who he is, what his purpose is, and how he fits into the world of demigods. This internal struggle could be a major part of the story, with Percy constantly fighting against the darker impulses that come from his new parentage while also trying to find a place where he can be accepted and still be himself in some way.
Well, Percy could start to resent Poseidon and all things related to him. He might decide to leave Camp Half - Blood and try to make it on his own in the mortal world. But then he realizes that his demigod powers still draw trouble, and he has to find a new way to deal with it without relying on his father's name.
In some fanfictions, if Percy Jackson becomes Death, he might gain the power to decide the fates of all beings. He could be seen as a very solemn and powerful figure, no longer the carefree demigod. His relationships with other characters would also change drastically. For example, his friends might fear him at first, but then come to understand his new role and the importance of it in the grand scheme of things.
Well, they might initially be shocked to see each other. The past Percy could be amazed at how much the future Percy has changed and grown. Maybe the future Percy tries to give the past Percy some advice to avoid future mistakes.
Perhaps in this fanfiction, Percy has special powers related to death. He could be able to communicate with the dead or have an understanding of the afterlife. This would add a whole new dimension to his character and the adventures he could have. Like, he might be called upon to solve mysteries that involve the spirits of the past.
Well, Percy could show off his water - controlling powers to the Avengers. Maybe he'd help Thor with some water - related Asgardian problem. Or he could use his skills to outwit Loki in a tricky situation.
One possibility is that it could lead to Percy's downfall. If Annabeth, who has been his close ally and friend, betrays him, it might shatter his trust not only in her but also in others. He could become more isolated and vulnerable, making it easier for his enemies to target him.
Well, in this fanfiction, Percy might have all kinds of adventures. He could be tested by the gods in strange ways. Maybe he has to prove his worth again, like facing some new and powerful monsters created by the gods just for him.
Well, perhaps Percy would initially mistake Thor for a new kind of demigod. Thor, on the other hand, would be intrigued by Percy's unique skills. They could exchange stories about their adventures. Percy would tell Thor about his quests in the Greek world, and Thor would share his tales of Asgard. And in the end, they might become friends and even learn new combat techniques from each other.