A great plot could involve Percy being hunted by other demigods who are jealous of his new status as the heir of Nyx. This would lead to a lot of action and conflict as he tries to prove himself worthy and protect his friends at the same time. He could also be sought after by the forces of the underworld who want to use his new powers for their own evil purposes.
Perhaps Percy has to go on a quest to find an ancient artifact that was once in Nyx's possession. Along the way, he meets new allies and enemies. He might have to face challenges related to Nyx's domain, like traversing through a realm of eternal night or battling creatures that thrive in the darkness. This would not only test his physical strength but also his mental and emotional fortitude as he comes to terms with his new role.
One idea could be Percy having to face a new type of monster that is related to Nyx's powers. For example, a shadow - creature that can only be defeated by using the unique abilities he inherits from being the son of Nyx.
One idea could be Percy's struggle with his new identity as Nyx's adopted son. He might face discrimination from some of the other demigods who are suspicious of his new connection. Another plot could involve Percy learning new powers from Nyx that set him apart in battles against monsters. He could also have to deal with Nyx's enemies who target him because of his relation to her.
Other demigods who are power - hungry could also be antagonists. They might envy Percy's new connection to Nyx and try to undermine him at every turn. They could form alliances against him and plot to steal his powers or his status as the heir.
One idea could be Percy having to face moral dilemmas as he discovers more about his connection to Kronos. For example, he might be tempted by the power Kronos once held. Another could be Percy having to deal with the distrust of his friends who fear he'll turn into an heir of Kronos in a bad way. He could also go on a quest to find a way to sever his connection to Kronos if it's a negative one.
Another interesting plot could involve Percy having to face off against other demigods who are jealous or afraid of his new status as the heir of Ouranos. They might form alliances against him, and Percy would have to use his wits and powers to overcome these challenges. He could also have to deal with the politics among the different godly factions who are interested in his new - found power.
One idea could be Percy discovers a new, hidden power within himself that is linked to an ancient prophecy. Maybe he can control the elements in a way no demigod has before.
Another plot could involve Percy being drawn into a quest related to the primordial world of his new parents. Maybe there's an ancient threat that only someone with his unique heritage can stop. This would take him to places like the Underworld in a whole new light, as he has a deeper connection to the primordial darkness there.
One possible plot for a 'Percy Jackson Champion of Nyx fanfic' would be that Nyx chooses Percy as her champion to stop an ancient evil that is rising. Percy would have to train in the ways of Nyx, which are very different from what he's used to. He'd have to learn to harness the power of the night, deal with Nyx's shadowy minions, and find a way to unite the demigods who are skeptical of his alliance with Nyx. As he delves deeper into this role, he uncovers secrets about Nyx's past and how it ties to the future of the demigod world.
One plot idea could be Percy struggling with his new identity. Since Tartarus and Nyx are such powerful and dark entities, Percy might have to battle against the dark urges within him. He could be shunned by his friends at first, but then prove that he's not defined by his parentage. For example, he might save Camp Half - Blood from a new, powerful threat using his unique powers that come from his heritage.
In a fanfic, perhaps Percy and his friends accidentally stumble upon Medusa's lair while on a quest. But instead of immediately attacking, Medusa offers them a deal. She says she will give them a valuable clue to complete their quest if they help her get revenge on one of the minor gods who wronged her in the past. Percy and his friends then have to decide whether to trust Medusa or not.