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.
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.
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.
His powers would be unique in that he could have a natural affinity for the Underworld's environment. He might be able to sense changes in the Underworld's energy, which could help him predict incoming threats from there. Also, he could have the power to manipulate the Underworld's minerals and gems, which could be used for various purposes, like creating powerful weapons or magical artifacts. This is different from normal demigods who usually have powers related to their Olympian parent's main domain, like a child of Poseidon having water - based powers.
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.
His powers would likely be much darker. He might have the ability to manipulate shadows like Nyx, using them to hide or even attack. And from Tartarus, he could have the power to cause earthquakes or open up chasms in the earth. He could also have a natural resistance to the harmful effects of the Underworld, such as the poisonous gases and the draining of energy that normal demigods experience.
His powers would likely be a blend of Greek and Norse elements. For example, he might still be able to control water like in his Greek demigod abilities, but also have some connection to ice or snow from the Norse side. Maybe he could freeze water in a way that's more related to Norse frost magic.
Percy, as the son of Bastet, would likely have powers related to feline - like abilities. For example, he might have enhanced speed, agility, and reflexes compared to other demigods. His senses, especially his sight and hearing, could be much sharper. Also, he could potentially have some sort of connection to cats, being able to communicate with them or understand their behavior in a way other demigods can't.
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.
He'd probably be more brooding. Growing up with Hades, he'd be exposed to the darker side of the mythological world constantly. So, instead of the somewhat upbeat Percy we know, he might be more withdrawn and moody.
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.