Perhaps the hotel ceiling has a portal to another dimension. Percy accidentally discovers this while staring at it. He then has to find a way to close the portal before strange and dangerous creatures from that dimension start pouring into the mortal world. He might have to use his powers and knowledge of Greek mythology to deal with this unexpected situation.
A plot could involve Percy staying in a hotel during a school trip. As he lies on his bed looking at the ceiling, he notices that the patterns on it seem to shift and form the face of a monster he has never seen before. This could start a whole new adventure where he has to figure out what the monster is and if it's a threat to him and his friends.
One plot idea could be that Percy, being a son of Poseidon, discovers he has Dragonborn abilities. He could use his water - controlling powers in combination with the Dragonborn's shouts. For example, he might use a frost shout while manipulating water to create icy spikes. Another idea is that Percy has to go on a quest in the world of Skyrim to find a magical artifact that can save both his world and the world of the Dragonborn from an ancient evil that threatens to destroy both realms.
One possible plot could be that the Avengers mistake Percy Jackson for a dangerous threat. Maybe they've been given false information about his powers being uncontrollable. So, they decide to kidnap him to keep the world safe. But once they have him, they start to realize that he's not what they thought. Percy, being a hero in his own right, tries to convince them that he's on the side of good.
One plot idea could be Percy having to team up with the children of Ares for a special quest. Maybe there's a powerful ancient weapon that only the combined strength of Poseidon's and Ares' descendants can wield. Another idea is Percy getting caught in the middle of a feud between the Ares cabin and another cabin, and he has to find a way to make peace. Or perhaps Percy discovers he has some unexpected connection to Ares' lineage that changes his perception of himself and his powers.
Well, a possible plot is that Percy has a flashback to a time in Tartarus when he witnessed a prophecy being fulfilled in a way he didn't expect. This could lead him to re - evaluate his understanding of prophecies in the present. Another idea is that his flashback reveals a secret passage or power source in Tartarus that he believes might exist in the mortal world too, and he sets out to find it. Also, the flashback could be about a time when Percy was separated from Annabeth in Tartarus for a brief moment, and that fear of losing her again triggers his current actions to protect her more fiercely.
One plot idea could be Percy having to team up with Hera to rescue a group of demigods who have been captured by an ancient monster. Hera, despite her usual aloofness, realizes she needs Percy's unique skills.
One plot idea could be that Percy's singing has magical properties. Maybe it can heal the wounded or calm the angry sea gods. So, he has to use his singing to save the day during a big crisis in the demigod world.
One idea could be Percy leading a new generation of demigods in a battle against an ancient evil that has been awakened in the future. Maybe he has to train them in new ways to combat the threats.
One idea could be that Percy uses his demigod skills in the courtroom. For example, his ability to read Ancient Greek could come in handy when dealing with old legal documents or in cases related to archaeology where Greek inscriptions are involved. He might also use his combat skills in a more metaphorical way, like being a fierce advocate who 'fights' for his clients.
In an Alaska - based Percy Jackson fanfiction, Percy could be on a quest that leads him to the frozen tundra. He might discover an ancient Greek artifact that has been hidden there for centuries. This artifact could hold the key to preventing a major disaster, like the awakening of an old and powerful enemy. Along the way, he could team up with some local Inuit demigods who have their own unique powers and knowledge of the Alaskan wilderness.