The character of Percy's younger brother in Zeus fanfiction could bring new dynamics. He could be a source of conflict, perhaps being jealous of Percy's achievements and trying to outdo him in the world of the fanfiction. Or he could be a support, working alongside Percy against common enemies. It all depends on how the author wants to develop the character.
One possible plot could be Percy, as the brother of Zeus, having to deal with the politics of Olympus. He might struggle to find his place among the other gods while also trying to use his powers for good. Maybe he teams up with some of the younger demigods to bring about change in the rather old - fashioned ways of the Olympian council.
Well, one possible plot could be Percy discovering he's Zeus' brother in a very unexpected way. Maybe through some ancient prophecy that was long forgotten. He'd then have to deal with the power struggles within the Olympian family, as well as his own identity crisis. Another aspect could be his relationships with other demigods changing, as being related to Zeus is a huge deal.
The main antagonists could be some of the more traditional and power - hungry gods. For example, Hera might see Percy as a threat to the order she has maintained for so long. She could try to undermine him at every turn, fearing that his new ideas and his relation to Zeus will disrupt her plans. Another antagonist could be Ares, who might be jealous of Percy's potential power and influence over the other gods and demigods.
A fanfic about Percy, brother of Zeus, could start with Percy feeling a sense of isolation. Despite being part of the divine family, he doesn't quite fit in. He decides to leave Olympus for a while and explore the mortal world. There, he experiences the joys and sorrows of humanity in a way that Zeus never has. He falls in love with a mortal, which causes a huge rift between him and Zeus. This love story could be the central theme of the fanfic, with Percy having to choose between his family and his new - found love.
Fanfiction often takes creative liberties. In this case, it might be that the author wanted to explore new power dynamics and character interactions. If Percy was Zeus' brother, it would completely change the power balance in the world of the series. Percy's connection to the sea and Zeus' control over the sky could lead to all sorts of interesting plotlines, like joint quests or power struggles between the two 'brothers'.
They could interact in many ways. For instance, Percy's younger brother might be a devotee of Zeus and constantly seek his guidance. Zeus, in turn, could use the younger brother for his own purposes, like having him perform tasks that Percy would refuse.
His powers would likely become more god - like. He might gain the ability to control lightning, like Zeus. Also, his strength and speed could be enhanced to a much greater degree. His control over water, which was his main power before, could also be amplified and perhaps combined with his new lightning powers in some unique ways.
He could have adventures in the sky realm. Maybe he would go on quests with Zeus' other children, like flying on thunderclouds to battle powerful sky - based monsters. He might also get involved in the politics of the Olympian gods, which would be a whole new kind of adventure compared to his usual sea - related ones.
In the fanfiction, Percy's relationship with Zeus may develop gradually. Zeus, being a powerful and often aloof figure, might initially be distant from Percy. But as Percy shows his courage and abilities, Zeus could start to take a closer interest. This might lead to Zeus mentoring Percy in some ways, teaching him about godly powers and responsibilities.