A great plot for 'Percy Jackson and Nemesis Fanfiction' could be that Nemesis plants seeds of doubt in Percy's mind about his past victories. She makes him question if he really deserved them or if he just got lucky. This causes Percy to go on a journey of self - discovery. He meets various characters from Greek mythology who help him understand the true nature of victory and defeat, and in the end, he learns to accept both as part of life.
One idea could be Percy having to face a series of challenges set by Nemesis that test his sense of fairness. For example, he might be put in situations where he has to choose between helping a friend who has made a mistake or following the strict rules. Another could be Nemesis creating an illusion of a perfect world for Percy, but he has to figure out that it's all a ruse and break free.
Well, perhaps in the fanfiction, Nemesis decides to target Percy's friends to get to him. So Percy has to make a deal with Nemesis to protect them. He might have to go on a quest to find a rare artifact that Nemesis desires. And during this quest, he discovers new powers within himself that are related to the concept of balance, which is what Nemesis represents.