A great plot idea is that the child has a special connection to the sea, like Percy, but also has Annabeth's wisdom. So they use this combination to solve a mystery that has been plaguing Camp Half - Blood for years. For example, the mystery of why some demigods' powers are fading. Another could be that the child gets sent back in time and has to interact with their parents when they were younger. This could create some really interesting situations and challenges for the child.
A possible plot could involve a magical tournament where Percy and Annabeth are on opposing teams. Their rivalry gets in the way at first, but as the tournament progresses and they see the real threats emerging, they start to cooperate in a more covert way to ensure the safety of everyone involved.
Another plot could revolve around the challenges of raising a demigod baby. Percy and Annabeth would have to protect the baby from various mythological threats. They might face monsters that are after the baby because of its potential power. Also, they would need to figure out how to educate the baby about their demigod heritage in a world that is both dangerous and full of wonder.
Another plot idea is that there's a rift in the House of Hades that is slowly corrupting the place and they have to find the source of it. Along the way, they face their own inner demons. Annabeth might struggle with her insecurities about being smart enough to solve the mystery, while Percy battles with his fear of not being strong enough to protect Annabeth. As they overcome these personal struggles, they also get closer to solving the problem of the rift.
One idea could be that Percy and Annabeth are on a pirate ship, and they discover a hidden treasure map. They have to battle other pirates and sea monsters to reach the treasure. Another could be that they are pirates who are trying to free a group of captured demigods. A third idea is that they are pirates on a quest to find a magical artifact that can control the seas.
One idea could be a post - quest moment where Percy and Annabeth are sitting by the Camp Half - Blood lake at sunset. They start sharing their deepest fears and hopes, and as they get closer emotionally, they hold hands and share a gentle kiss. It's a very tender and intimate moment that shows their deep connection.
Well, a great plot could be that they find a mysterious object under their bed. It turns out to be a key to an ancient demigod treasure. So they start researching in bed, looking through old scrolls and books, trying to figure out where this treasure might be. And during this process, they also share some sweet and tender moments, reminiscing about their past adventures. Also, they could be in bed during a stormy night at Camp Half - Blood and tell each other spooky stories about the monsters they've faced, which leads to them coming up with new strategies for future battles.
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).
Maybe they are rivals in a quest given by the gods. Each wants to prove themselves as the better hero. Percy uses his combat skills and connection to the water, while Annabeth uses her strategic mind and knowledge of ancient places. This could lead to some intense moments where they have to decide whether to cooperate or keep competing.
Well, they could face issues like how to balance their heroic duties with their married life. For instance, Percy might get called away on a quest right when they planned a romantic dinner. Or perhaps in the fanfiction, they start a school for demigod children together as a married couple, which would bring its own set of comical and heartwarming situations.
They could discover a lost civilization in Tartarus. This civilization might have a way to manipulate the very fabric of Tartarus itself. Percy and Annabeth would then have to learn the ways of this civilization in order to use it to their advantage. Maybe they have to earn the trust of the civilization's leaders by helping them with their own internal problems first.