In fanfiction, Percy could absorb the ability to heal others instantly. This would be a great addition to his existing powers as it would make him not just a fighter but also a healer. Additionally, he could absorb the power to communicate with animals. This would give him an edge in many situations, like getting help from wild animals during a battle or getting information from them about the surrounding area.
One idea could be that Percy can control not just water, but also the temperature of it. So he can freeze water instantly to create ice weapons or barriers. Another could be that he has enhanced underwater vision, allowing him to see clearly even in the darkest and murkiest depths, which would be really useful in his underwater adventures.
He could absorb the Sharingan ability. Imagine Naruto with the power to copy jutsus just like a Uchiha. Another interesting one would be the Byakugan. He could gain enhanced vision and be able to see the chakra points even more clearly. Or perhaps the ability to control sand like Gaara. That would be really cool.
In the fanfiction, the power of fire manipulation could be really interesting for Percy to absorb. Since he is mainly associated with water, having the power of fire would create an internal struggle between his original element and the new one. Also, the power of teleportation would be cool. He could instantly travel to different places, which would make the story's pacing much faster and more exciting. And the power of mind - reading would give him an edge in understanding his enemies' plans and his friends' true feelings.
One possible plotline could be Percy teaching his demigod child about their powers and the world of the gods. For example, he might take them to Camp Half - Blood and introduce them to the cabins, the training arenas, and the other demigods. Another plot could involve Percy protecting his family from a new threat, like a powerful monster that has a vendetta against him. Or perhaps his child discovers a new and unknown power, and Percy has to help them understand and control it.
Another ability could be 'Inventory Management'. The OC can access a virtual inventory like in a role - playing game. They can store items in it, and when needed, pull out useful things such as magical weapons or potions. It's like having a portable storage space that only they can access, which gives them an edge in battles and quests.
One new ability could be ice - skating at super - speeds. He could glide across water or land, using his ice - skates made of pure ice, and quickly reach his destination or dodge attacks. It would be like a combination of his natural athleticism and his new ice powers.
Maybe in the fanfiction, the zoo is a place where the gods have hidden some of their lost magical artifacts. Percy, being the son of Poseidon, senses a strange water - based magic near the aquarium section. He starts to investigate with Annabeth. Along the way, they face challenges from zoo security (who are unknowingly being influenced by a minor god who doesn't want the artifacts found) and various magical traps set around the exhibits.
A plot could involve Percy being sent to Forks on a quest. He discovers that there's a powerful artifact hidden in the area that could cause chaos if it falls into the wrong hands. The vampires and werewolves in Forks are also after this artifact for their own reasons. Percy has to navigate the complex relationships between these two groups while also trying to keep his true identity as a demigod a secret. Along the way, he forms an unlikely friendship with Jacob, who helps him understand the ways of the werewolves.
One possible plot could be Percy arriving in the DxD world and being mistaken for a new, extremely powerful devil. He then has to navigate the complex politics of the devils while also trying to find a way back home. Along the way, he uses his demigod powers in unique ways against the various threats in the DxD universe.
If Percy Jackson were a vampire in a fanfiction, he might retain his water - controlling abilities from being a demigod. His vampire strength could be combined with his natural athleticism, making him an even more formidable fighter. He could use his speed to quickly reach bodies of water to enhance his powers, and his vampire senses might help him detect threats in the water as well.