Another plot could involve Percy being frozen as part of a time - stopping spell. While he's frozen, the world around him starts to change in strange ways. The other demigods notice that monsters are becoming more powerful and they realize they need Percy to come back to set things right. So they try to figure out how to thaw him out by delving into ancient prophecies.
One idea could be that Percy is frozen in ice due to a powerful curse from an ancient enemy. His friends in Camp Half - Blood have to go on a quest to find a magical item that can break the curse. Maybe it's a rare flower that only blooms in a certain realm.
One idea could be Percy having to journey into a frozen realm in the Underworld to retrieve a magical ice artifact that is key to saving Camp Half - Blood from a new threat. Maybe he teams up with a frost - giant who has a change of heart and helps him.
One idea could be Percy having to control his new ice powers while facing old enemies. For example, when fighting monsters like the Minotaur, he could use his ice powers to freeze the ground beneath it, making it difficult for the Minotaur to move. Another plot idea is Percy trying to understand his new role as a primordial. He might seek out ancient beings or gods who could give him more information about his powers and his place in the universe. He could also face challenges from other primordial beings who see him as a threat or an upstart in their realm of power.
A powerful ice giant could be the antagonist. This ice giant could have a grudge against Percy for some past encounter and uses its ice - magic to freeze him. It wants to keep Percy out of the way while it tries to invade Camp Half - Blood or other places important to the demigods.
In a Percy Jackson - Frozen fanfiction, perhaps Annabeth could meet Anna. Annabeth would be impressed by Anna's determination and loyalty. They could go on an adventure in the Arendelle kingdom, with Annabeth using her strategic mind to solve problems that the Frozen world is facing, like a strange curse that is making the ice in the kingdom melt uncontrollably.
Some interesting Frozen 2 fanfiction plot ideas could revolve around the idea of a magical item. For example, there could be a magical crystal hidden deep in the forest that has the power to either save or destroy both the forest and Arendelle. Elsa and her friends embark on a quest to find this crystal before it falls into the wrong hands. Another idea is that a dark force from outside the known world tries to invade the forest and Arendelle, and the characters have to unite not only the people of Arendelle and the forest but also other magical beings to fight against it. Also, a plot could be centered around a prophecy that was never fully revealed in the movie, and Elsa and Anna must figure out what it means for their future.
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 interesting plot twist could be that Percy's water powers somehow interact with Elsa's ice powers in unexpected ways. Maybe when they meet, their powers combine to create a new form of frozen water element that has unique properties. Another could be that the demigods from Percy Jackson's world have to deal with the magical cold of Arendelle, which is a whole new kind of challenge compared to their usual Greek - related threats.
One idea could be that Percy discovers his ice powers during a dangerous mission. Maybe he's in a really hot and hostile environment, like a volcanic area, and suddenly his ice powers emerge to cool things down and save his friends. Another could be that he has to train with an ancient ice spirit to control his new powers. The spirit could be really strict and Percy has to learn discipline along with mastering his ice abilities.
Well, a cool plot might be that there's a merman who has a special power to control water currents, and Percy needs his help to find a lost magical artifact deep in the ocean. In return, Percy has to help the merman get accepted by the surface - dwelling gods. Or perhaps a merman is wrongly accused of stealing Poseidon's trident and Percy has to prove his innocence.
One idea could be Percy having to face a new threat underwater that only Poseidon can help him understand. Maybe a lost civilization that Poseidon once had a connection with is now causing trouble. Another could be Percy going on a quest to find a powerful artifact that was once Poseidon's but has been stolen, and Poseidon gives him special powers or guidance along the way.