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.
Some of the gods might also be important characters. Poseidon, Percy's father, could have an interesting reaction to Percy's new relationship. Hades, as the god of the underworld where Styx is located, might also be involved, either as an ally or an obstacle depending on the direction of the fanfiction.
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.
One plotline could be the overprotective brother constantly trying to keep Percy out of dangerous quests. He might try to interfere with Chiron's plans for Percy, thinking he's too young or inexperienced. Another could be the brother getting into arguments with Percy's friends like Annabeth when he feels they're leading Percy into harm's way. And there could also be a plot where the brother himself gets involved in a quest to protect Percy and discovers his own hidden powers along the way.
In a fanfiction of Percy Jackson dipping in the Styx in Tartarus, it might explore the idea of him getting new powers or a stronger connection to his godly side. For example, his water - controlling abilities could become more refined and he could have a better understanding of the Underworld's waters. Also, he could encounter some long - forgotten spirits or creatures that guard the Styx in Tartarus.
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.
One interesting plotline could be the Seven facing a new, more powerful enemy that is a combination of different mythological creatures. This would test their teamwork and individual powers in new ways.