In some fanfiction, a cool plotline is that Percy and Annabeth discover a hidden part of Camp Half - Blood that has been sealed away for centuries. This place holds secrets about their parents and their own destinies. As they explore, they encounter old magic and ancient creatures. They also have to deal with the fact that some people in the camp don't want them to uncover these secrets. It creates a lot of tension and mystery, and of course, their relationship is put to the test as they face these challenges together.
Well, a great plotline might be Percy and Annabeth getting separated during a battle and each having to face their own set of challenges while trying to find their way back to each other. Maybe Percy ends up in a strange part of the Underworld and has to navigate it alone, while Annabeth is stuck on an island full of dangerous monsters. They both grow stronger individually and when they reunite, they are even more of a power couple. Also, a plotline where they are chosen to train a group of new demigods and they face difficulties in teaching them the ways of the camp could be really engaging.
One interesting plotline could be Percy and Annabeth going on a quest outside of Camp Half - Blood to find a long - lost artifact that has the power to protect the camp from a new, unknown threat. Another could be them having to deal with a rift within the camp that divides the demigods, and they have to work together to reunite everyone. And there could also be a plotline where they time - travel back to the early days of the camp to prevent a disaster that could change the future of the camp forever.