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.
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.
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.
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 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.
Maybe a plot where Dark Percy has some sort of power that Thalia needs to save someone she loves. Thalia has to convince Percy to use his powers for good in this case, and in the process, they develop a complex relationship. Percy is conflicted between his dark side and the pull he feels towards Thalia's cause.
Another plot could be that Percy and Thalia get trapped in a magical realm that is a mix of their worst nightmares. In order to escape, they have to face their fears head - on. This forces them to open up to each other about their insecurities and past traumas. As they work together to find a way out, their bond grows stronger. They might also meet some strange creatures in this realm that give them clues about how to break free, and they have to use their wits and powers to decipher these clues.
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.
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.