A plot could involve Percy's friends being in danger and the only way to save them is by making a deal with the entities in the Styx. He might have to sacrifice something precious, like a part of his powers, in order to gain their help. This would create a great internal conflict for Percy as he weighs the cost of saving his friends against his own abilities.
Well, a plot could involve Percy being lured into Lady Styx's domain by a false prophecy. Once there, he finds out that Lady Styx has a plan to flood the mortal world with the waters of the Styx, which would bring death and destruction. Percy has to find a way to stop her, maybe by finding an ancient artifact hidden in the Underworld that has the power to control the Styx. Along the way, he meets some strange and dangerous creatures that are loyal to Lady Styx.
Well, perhaps Percy in the fanfiction is faced with a moral dilemma related to his role as Champion of Styx. For example, he might be asked to make a sacrifice that goes against his principles but is necessary to save the world. Or he could be dealing with the jealousy and suspicion of his friends who don't fully understand his new powers and status. This could lead to some really interesting character development and story arcs.
Perhaps in the fanfiction, Percy's dip in the Styx could be interrupted by an unexpected enemy. This could lead to him having to fight while in a vulnerable state, and his friends having to come to his rescue. It could add a whole new level of danger and excitement to the story.
Perhaps a plot where Lady Styx reveals some long - hidden secrets about Percy's past or his family. This could lead to Percy going on a journey of self - discovery while also dealing with the implications of these secrets in the world of demigods.
A really interesting plot idea could be that the blessing by the Styx actually attracts unwanted attention from the gods. Some gods might be jealous of Percy's new - found power and try to undermine him. He then has to navigate the tricky politics among the gods while also dealing with his own new abilities. Maybe there's a prophecy that's related to his Styx - blessed state and he has to fulfill it, but there are many obstacles in his way, both from the mortal and divine realms.
Another interesting plot could be Percy's internal struggle. He's used to being Poseidon's son, and now this new connection to Styx throws his identity into a bit of chaos. He has to figure out how to balance his old self with this new aspect of his being, all while facing various quests and threats in the Percy Jackson universe.
One plot idea could be Percy having to team up with Hera to rescue a group of demigods who have been captured by an ancient monster. Hera, despite her usual aloofness, realizes she needs Percy's unique skills.
One idea could be Percy and his friends getting trapped in a virtual game world that's a twisted version of their own. They have to use their demigod skills to defeat the digital monsters and find a way out.
Maybe Percy is sent on a quest to Atlantis because there are rumors of a traitor among the Atlanteans who plans to side with the Titans. Percy has to navigate the complex political situation in Atlantis and figure out who the traitor is before it's too late. He also has to deal with the Atlantean magic which is different from what he's used to.
One idea could be that Percy discovers a hidden realm within the world of the Wanderer. This realm has its own set of rules and challenges that he has to navigate. Maybe he meets new demigods there who have been lost for centuries.