It could be some of the jealous gods. Since Percy, Thalia, and Nico are living on Olympus, which is quite an unusual thing for demigods, some gods might not like their presence and try to get rid of them. For example, a god who feels that their power is being threatened by these demigods.
Well, one possibility could be that they are given special status on Olympus. Maybe they are tasked with keeping the peace among the gods, which would lead to a lot of interesting interactions with the deities. They might also use their unique powers to improve the living conditions on Olympus, like Percy controlling water to create beautiful fountains or something.
The fanfiction could also focus on their relationships. Percy, Thalia, and Nico might form a very tight - knit group, almost like a family. They would have to rely on each other in the often - cutthroat environment of Olympus. Their friendships could be tested as they grow older and face different challenges. For instance, Percy might be offered a high position in Poseidon's court that would take him away from his friends. Thalia could be given responsibilities that conflict with her loyalty to her friends. Nico might struggle with his identity as he tries to fit in on Olympus while still maintaining his connection to the Underworld.
In some fanfictions, it could be an ancient and powerful Titan who has escaped from Tartarus and is seeking revenge on the demigods. This Titan might see Percy and Thalia as the main obstacles in his path to domination.
Nico, he's that complex character from the Percy Jackson universe. He has a connection to the dead and often brings a sense of mystery. Thalia, she's a strong and fearless character, being a child of Zeus. And then there's Percy. He's the hero we all know, with his loyalty and his amazing water - controlling powers. In the Alaska fanfiction, these three characters are likely to be in the center of the story, having their own arcs and interactions.
The Olympians themselves could be the main antagonists in a sense. Since Percy is betraying them, they would be the ones trying to stop him. Zeus, with his power and pride, would likely be at the forefront, seeing Percy's betrayal as a direct threat to his authority.
It could be the agents of Order who see Percy and Thalia joining Chaos as a threat to the balance they strive to maintain. They might send their best warriors to stop Percy and Thalia from carrying out Chaos' plans.
A possible plot twist could be that one of them is actually a secret weapon of an enemy god. For example, Nico could be unknowingly influenced by a dark force that wants to use his powers to bring chaos to Olympus.
Percy could develop a more god - like sense of responsibility. Growing up on Olympus, he would be constantly exposed to the grandeur and the weight of the gods' duties. Thalia might become more confident in her leadership abilities as she is among the most powerful beings from the start. Nico could overcome his shyness faster as he is used to the presence of the gods. He would be more assertive in using his powers.
They could be like siblings. Percy and Thalia might act as the older siblings, protecting and guiding Nico. Percy, with his leadership skills, and Thalia, with her combat abilities, would be great at looking out for the younger Nico. They could also be like cousins, with a close - knit bond but still having their own individual family heritages that sometimes create interesting dynamics.
Perhaps a new character introduced in the fanfiction. A seemingly friendly newcomer who infiltrates their group, gains their trust, and then betrays them at a crucial moment. This character could be working for an enemy force, either from the Titans or some other ancient power that wants to see the downfall of Percy, Thalia and Nico.