Percy Jackson's power over water could be really interesting when combined with Diana Prince's combat skills. For example, Percy could create water barriers or even water weapons for Diana to use in battle. His ability to control the sea could also be used to quickly transport them across large bodies of water during their adventures together.
One plot idea could be that Percy Jackson, while on a quest, accidentally stumbles into Diana Prince's world. They team up to fight a common enemy that threatens both the demigod and the Amazonian worlds. Maybe a powerful ancient evil that has the ability to disrupt the balance between the mortal and the divine in both their realms.
Annabeth could be a strong ally. She has always been there for Percy, and even though his parentage has changed, her loyalty might still hold. Grover too. He's Percy's best friend and would likely stand by him no matter what. Another potential ally could be the Hunters of Artemis. Since Diana is another name for Artemis, they might see Percy as someone they need to protect and help, especially if he is involved in something that concerns the goddess.
One great 'Percy Jackson x Diana fanfiction' is 'The Meeting of the Demigods'. It has an interesting plot where Percy and Diana first cross paths during a quest. The story does a great job of portraying their different powers and personalities.
A common theme is the clash of different mythologies. Percy comes from Greek mythology and Diana from Roman mythology. Fanfictions often explore how these two different mythological backgrounds interact.
One interesting plotline could be Percy having to deal with new powers inherited from Diana. For example, he might have an enhanced connection to nature and animals, which could lead to adventures where he has to communicate with forest spirits to save his friends. Another plotline could involve Percy facing off against a new set of enemies who are specifically targeting him because of his relation to Diana. These enemies might be ancient foes of Diana's that now see Percy as a threat to their plans.
One great 'Percy Jackson Sea Prince Fanfiction' is 'The Sea Prince's Ascent'. It really delves into Percy's newfound powers as a sea prince. The author does an amazing job of describing how Percy navigates the complex underwater politics. Another good one is 'Percy, the Sea Prince and the Quest for Atlantis'. It combines the adventure elements typical of the Percy Jackson series with Percy's role as a prince of the sea.
Grover can also be a great character in these fanfictions. His friendship with Percy is always a key element. When Percy becomes a prince, Grover might be the one to keep him grounded and remind him of his roots. He could also be involved in some comical situations as he tries to adjust to Percy's new royal lifestyle, like accidentally causing a scene at a grand ball or getting lost in a huge royal palace.
One idea could be a story where Michael Jackson and Diana Ross collaborate on a huge musical project. They start by sharing their different musical visions and gradually combine their styles to create a new genre of music that takes the world by storm.