In a fanfiction where Percy is the son of Hades and Poseidon, his powers would be truly extraordinary. He could have the ability to create portals in water that lead to the Underworld. This would give him an advantage in both battle and travel. His hydrokinesis could be tainted with the essence of death, so when he attacks with water, it could cause a draining effect on his opponents, sapping their life force. Additionally, he might be able to sense the presence of the dead in any water source, which could be used for both reconnaissance and protection.
Percy's powers would be more complex. He could have the ability to communicate with sea deities in a different way. Maybe he could use Hera's diplomatic skills to make deals with them or resolve disputes. His control over water could also be extended to other substances that have a connection to water, like steam or ice, in a more efficient way than before. And his powers of persuasion, which could be influenced by Hera, might be used to calm angry sea storms or make sea monsters cooperate with him, which is an interesting combination of his Poseidon - given powers and Hera - influenced skills.
Well, if Percy was the son of Loki and Poseidon in fanfiction, his powers would be a really wild mix. He might have Poseidon's control over water, but with a Loki - like trickster twist. For example, he could create illusions in the water, making it seem like there are sea monsters when there are none to deceive his enemies. He could also use Loki's shape - shifting ability while in the water, perhaps turning into a mermaid - like form with a mischievous glint in his eyes. His power over water could also be used in more devious ways, like creating whirlpools that suck in his foes and then spit them out in a different location, all while laughing like Loki would.
Percy could have the power to manipulate the coldness that is often associated with the Underworld. He could freeze his opponents or create ice - like structures out of the cold energy. This would be different from his water - based powers in the normal story. Also, he may have a stronger connection to the gems and precious metals that are found in the Underworld, being able to sense them and even use them in some way for his benefit.
His water - based powers might be more intense. Since he's directly related to Poseidon, he could have greater control over large bodies of water, like being able to create huge tidal waves with just a thought, instead of just small ones. He might also be able to communicate with sea creatures on a deeper level, understanding their complex languages and using them to his advantage in ways he couldn't before.
Well, in the original Percy Jackson stories, Percy has powers mainly related to water as the son of Poseidon. But in this fanfiction with Hestia as his mother too, he might have an added connection to fire or the concept of home. So, he could potentially have the ability to control the temperature of water, like making it boil or freeze instantly, which combines his water powers with a bit of the hearth - related powers. He might also be able to create a sort of protective shield around himself and his friends that has the properties of both the ocean's strength and the warmth and safety of the hearth.
If Percy were the son of Poseidon and Hera, his powers would be quite unique. His water powers could be enhanced in a different way. Maybe instead of just controlling water, he could transform water into different substances related to Hera's domain, like turning water into a fertile soil - like substance for plants to grow. His leadership skills would be off the charts. He could use his Poseidon - given strength and Hera - influenced charisma to lead not just demigods, but also other mythical creatures. He might also have the power to protect a place or a group of people in a more profound way, like creating a shield made of both water and a golden energy that represents Hera's protection.
Well, instead of summoning water or having a connection with the sea, he might have the power to shift shapes, a common ability associated with Loki. He could turn into different animals or even other people to gain an advantage in a fight or to infiltrate an enemy's stronghold. Also, his power over fire could be enhanced, as Loki has some association with fire in Norse mythology. He could use this to create distractions or attack opponents from a distance.
One possible plot could be Percy struggling with the duality of his parentage. He might face discrimination from both the children of Hades and Poseidon. For example, the children of Poseidon could see him as a threat because of his connection to Hades, and vice versa. He could then go on a journey to prove that his dual heritage doesn't define him, but rather gives him unique powers to protect the world of the gods and mortals.
One great fanfiction idea could be a story where Zeus, Poseidon and Hades have to team up to face a common threat that is even more powerful than them. Maybe a primordial being awakens and they must put aside their differences. In this story, Percy could be the mediator between them, using his unique position as the son of Poseidon to try and keep the peace among the three powerful gods.
In fanfiction, Percy's connection to Poseidon could be deepened, allowing him to draw on the power of the entire ocean instead of just the water around him. This would mean he could cause tsunamis on the other side of the world to help in a situation far from his location. It would also give him access to more knowledge about the ocean's secrets, which could be used to solve mysteries.