The gods themselves could be interesting characters to include. For example, Poseidon could offer Percy more power or advice at crucial moments. Athena could also play a role, perhaps initially being an adversary because of her different goals, but later becoming an ally. Even minor gods like Hecate could be included. Her magic could be used to help Percy in situations where his normal powers aren't enough. Also, new characters created specifically for the fanfiction, like a powerful demigod from a previously unheard - of lineage, could add an element of surprise and freshness to the story.
Grover could be an interesting addition. Even though he's a satyr, his loyalty to Percy and his own unique skills could be put to good use in a sea - centered story. Maybe Nereids, the sea nymphs. They can bring an element of mystery and beauty to the fanfiction and might be able to help Percy in some way. Another option is Chiron, who could guide Percy from the shore in his sea - related quests.
Definitely include Percy and Annabeth. They are the central characters in the original series and their relationship and skills are key. Percy with his water - related powers and Annabeth with her wisdom and strategic thinking.
Annabeth Chase would be a great character to include. She is smart and can help Percy with strategizing in difficult situations. Her knowledge of architecture and ancient history can come in handy when they are exploring new places in the ocean world.
Annabeth would be a great character to include. She could use her intelligence to help Percy solve problems and strategize against their enemies in the underwater world.
Well, Percy himself is of course the main character. Annabeth would also be very likely to be there. Their relationship is a big part of the Percy Jackson series, and in a Tartarus - related fanfiction, she could be either with Percy from the start or trying to find a way to rescue him. Grover might also show up. His loyalty to Percy and his skills as a satyr could be useful in the dangerous environment of Tartarus.
A plot idea could be that Percy discovers a hidden underwater civilization that has been in stasis for centuries. This civilization has a powerful weapon that could either save or destroy the world. Percy has to navigate the politics and power struggles within this new society to either obtain the weapon for the good of the world or stop someone else from using it for evil. Also, Percy could lose his memory and end up in a normal human town. He has to rediscover his powers while also protecting the town from a series of mysterious attacks that are actually related to the world of the gods.
Jason Grace is often included. His leadership skills and ability to fly would be useful in various crossovers. In a crossover with the X - Men, he could be seen as a mutant with his flying and lightning powers. Piper McLean is also a favorite. Her charm - speak ability could be very effective in different crossovers. For instance, in a crossover with the world of Game of Thrones, she could use it to manipulate some of the power - hungry lords. And let's not forget Grover. His satyr nature and his loyalty to Percy make him a great addition to any crossover.
Grover is a great character to include too. His connection to nature and his loyalty to Percy make him a wonderful addition. He can provide comic relief at times, but also be a crucial part of the team when it comes to using his nature - based powers to sense danger or find hidden paths.
Annabeth is a great character to include. She's smart, strategic, and her relationship with Percy adds a lot of depth. Grover is also good. His loyalty and his connection to nature can bring interesting storylines. Nico di Angelo can be a really cool addition too. His powers over the dead and his somewhat brooding personality can create some great drama.
Well, obviously Percy and Phoebe are the main ones. But other great characters to include could be Annabeth. She could be jealous of Phoebe at first, creating some interesting drama. Grover would also be a good addition. His ability to sense danger and his friendship with Percy could play an important role in the story.