One idea could be that in the alternate universe (au), Thalia never becomes a tree. Instead, she and Percy go on a quest together to find a powerful artifact that can stop an ancient evil from awakening. They face many challenges along the way, like battling mythical creatures that are stronger than usual due to the dark magic seeping into the world.
Another plot could be that Percy and Thalia are transported to a modern-day city in the au, but they still have their powers. They have to figure out how to blend in while also fighting off monsters that are attracted to the large population. They form a new kind of friendship as they rely on each other in this strange new environment, and they also meet new demigods who have unique abilities and help them on their adventures.
Maybe in the au, Percy and Thalia are from different camps that are at war with each other. But they discover that they are being manipulated by a greater force. They then team up secretly to find out the truth and end up falling in love during their dangerous journey of uncovering secrets. Their relationship causes a stir in both camps as they try to bring peace.
Perhaps a fanfiction where Percy and Thalia are transported to an alternate dimension. In this new world, they have to figure out how to get back while also dealing with different versions of their friends and enemies. Thalia might have to face an alternate version of herself who has turned evil, and Percy could be the one to help her see the light again. It would be full of action, drama, and self - discovery.
Perhaps the prophecy involves Thalia having to team up with an unexpected ally. For example, a former enemy who has also been touched by the prophecy in some way. They would have to overcome their differences and learn to trust each other while facing numerous challenges like dangerous quests and battles against powerful Titans or other mythical foes.
One idea could be that Percy and Thalia have to go on a secret quest together, away from the rest of the demigods. During the quest, they discover new powers within themselves and also realize their feelings for each other are deeper than just friendship. Another could be a story where they are trapped in a magical realm and have to rely on each other's skills to escape, with lots of action and some tender moments in between.
One idea could be exploring how their demigod powers are affected during the pregnancy. For example, Percy's control over water might become more erratic due to hormonal changes. Another could be the reactions of their friends and the demigod community. Some might be supportive, while others might fear for the future of the half - bloods. Also, they could face challenges from the gods, as the birth of a new demigod with such powerful parents could shift the power balance in the mythological world.
One idea could be a pre - series fanfic where Luke, Thalia and Percy's parents all interact. Maybe they are on a quest together before the whole 'Big Three' pact drama. Another could be a fanfic where Luke and Thalia have to rescue Percy from a new and powerful monster that even he can't handle on his own. It could also be interesting to see a fanfic where they all time - travel back to an important moment in the history of the gods and have to change something to save the future.
One idea could be a modern-day high school AU. Percy could be the new kid at school, a bit of a loner and a troublemaker. Annabeth, on the other hand, is the top student and part of the student council. They get paired up for a project and initially clash because of their different personalities. But as they work together, they discover they have more in common than they thought, like their love for the ocean (Percy's always been drawn to it, and Annabeth has a fascination with ancient sea civilizations).
Annabeth would be a great addition. Her intelligence and strategic thinking could add a lot to the story. For example, in a situation where Percy and Thalia are stuck in a maze - like the Labyrinth - Annabeth's knowledge of architecture and mythology could help them find their way out. She could also create some interesting relationship dynamics, like a friendly rivalry with Thalia.
A plot could revolve around them having to protect a young demigod who is being hunted by a powerful enemy. Percy and Thalia have different approaches to protection, and they might argue at first. But as they face more danger, they realize that they need to work together. Their sibling relationship would grow stronger as they defend the young demigod.
One idea could be that they discover a long - lost family heirloom together. They go on a quest to figure out its significance and along the way, they face various monsters and challenges. It could strengthen their bond as siblings.
One idea could be Percy and Thalia teaming up to protect a group of young demigods who are being hunted by a new and powerful monster. They could use their combined skills, Percy's water powers and Thalia's electric ones, to fend off the attacks.