Another plot could involve Percy trying to find his place in a world where the Romanov legacy still looms large. He might be torn between his modern life and the expectations and traditions that come with being part of the Romanov family. He could meet other descendants or figures related to the Romanovs and form unexpected alliances or rivalries.
Perhaps in the fanfiction, Percy discovers that his Romanov heritage gives him some unique powers or abilities. He then has to learn how to control and use these powers while also hiding his true nature from the outside world that might not understand or might try to take advantage of him.
Obviously, Percy is one of the main characters. There might also be other characters from the Percy Jackson universe who interact with him in this new context of being the son of Romanov. For example, his friends like Annabeth could be there to support him or question his new identity.
One interesting plot could be Percy having to face off against Medusa's other children in a battle of wits and strength. Maybe they are out to prove themselves as worthy as Percy, who has the fame of being Poseidon's son. Another could be Percy having to deal with the stigma of being 'Son of Medusa' in a world that fears her so much. He might have to constantly prove he's not a monster like his 'mother' is perceived.
Perhaps a plot where Percy is summoned by Khione to help her with a problem in her icy realm. There, he meets other children of Khione and they have to work together to stop a dark force that is melting the ice in her domain. Percy uses his skills as a demigod and his unique heritage to come up with a plan to save the day.
In some fanfictions, there could be a plot where Percy is faced with a moral dilemma due to his connection to the 'Son of the Pit'. Maybe he has to decide between using a dark power within him to save his friends or resisting it and finding another way, which would show his growth and the struggle between his good nature and the darker aspects of his heritage.
One interesting plot could be Percy's struggle with his new identity as the son of death. He might have to deal with being feared by others in the demigod community. For example, he could face challenges when trying to form alliances as other demigods are superstitious about his parentage.
One interesting plot could be the adopted son discovering his own unique powers that are different from the typical demigod powers in the Percy Jackson world. Maybe he has a special connection to an ancient and forgotten god, which leads to all sorts of adventures as he tries to figure out what it means and how to control it.
One interesting plot could be Percy's adventures in the underwater realm as the son of Amphitrite. He might discover new sea creatures and magical underwater places. Another could be his interactions with other demigods from a different perspective, since his mother is Amphitrite.
One interesting plot could be Percy discovering new powers related to being the'son of void'. For example, he might be able to manipulate dark energy in ways that help him in battles against new and powerful enemies. Maybe he can create portals to the void to escape or surprise his foes.
Maybe there's a plot where Percy has to team up with other demigods who also have primordial roots. Together, they have to face a great evil that is threatening to destroy the balance between the primordial and Olympian worlds. They would have to use their combined powers and knowledge to save both worlds.
Perhaps there's a plot where Percy forms an alliance with a dragon. He has to earn the dragon's trust, and together they go on a quest. The dragon might be the key to unlocking some ancient mystery that has been hidden for centuries, and Percy's unique heritage as the 'son of a dragon' plays a crucial role in this journey.