One possible antagonist could be an ancient and powerful Titan who has escaped from Tartarus. This Titan is bent on revenge against the gods and sees Percy and Jason as the key to getting to the Olympians. He uses his dark magic to create all sorts of disasters and tries to turn Percy and Jason against each other. Another could be a group of rogue demigods who have turned to the dark side. They envy Percy and Jason's status and friendship and try to undermine them at every turn.
One idea could be that Percy and Jason have to team up to save a group of demigods who are trapped in an ancient, cursed maze. They use their combined powers of water and lightning to break through the various obstacles. Another could be a story where they swap powers for a day and have to deal with the consequences. For example, Percy tries to control lightning but accidentally zaps some of their friends' hair into crazy styles, while Jason struggles to keep the water from going wild when he tries to use Percy's power.
One great fanfiction is 'The Adventures of Nico, Percy and Jason'. It really captures their unique personalities well. The author does an amazing job of putting them in new and exciting situations that feel true to the original series.
It's hard to name just a few as the best, but some authors who are quite well - known in the fanfiction community for their Percy and Jason works are those who really capture the essence of the characters. There's this one author, whose username is AquaticWriter. Their stories often have really detailed descriptions of Percy's water powers and how they interact with Jason's lightning - based abilities. Another one is RomanHeroFan. They are great at writing the dynamic between Percy and Jason, especially when it comes to their cultural differences from Greek and Roman backgrounds.
There's also 'FanficHero'. This author is great at creating unique storylines for Percy, Nico, and Jason. In one of their fanfictions, the three are transported to a different dimension and have to find their way back while also dealing with new and strange threats. Their writing style is very engaging and keeps the readers hooked from start to finish.
There is a fanfiction called 'Jason, Percy and the Lost Prophecy'. This one is quite long but really engaging. It has a complex plot where Jason and Percy are on a mission to find a lost prophecy that could change the fate of the demigod world. The way the two characters interact and grow throughout the story makes it a must - read for fans of both characters.
There's also 'Jason and Percy: United Front'. In this fanfic, the two characters are forced to work together despite their initial differences. It delves deep into their personalities and how they learn to complement each other. The writing style is engaging and it keeps you hooked from start to finish. It also includes some cool new interpretations of the demigod powers.
Annabeth could be a great character to include. She could be a friend to both of them and initially be a bit confused or even jealous of their growing relationship. Her presence can add some drama and also show the different dynamics between the characters.
One great 'Percy x Nico x Jason' fanfiction is 'The Three Heroes' Journey'. It beautifully weaves their characters together, exploring their friendships and the challenges they face as a trio. The author does an amazing job of capturing their individual personalities. Percy's loyalty, Nico's mystery, and Jason's leadership all shine through in the story.
I'd recommend 'The Betrayed Friends'. In this one, there's a twist where one of Percy's so - called friends turns out to be working against them. It makes for a really interesting read as you wonder who it could be. Another is 'Friends or Foes?' where the line between friendship and enmity gets blurred when they encounter a group of characters with a strange agenda. The twist here keeps you on the edge of your seat.