One interesting plot could be that Percy has to face an ancient and powerful Titan that has been awakened from a long slumber. He uses his unique water powers in unexpected ways, like creating a giant water vortex to trap the Titan and then using his sword to deliver the final blow. Maybe he also teams up with some unlikely allies, such as a group of minor gods who have a vendetta against the Titans.
Another plot could involve Percy having to kill a Titan in a different realm. He gets transported to a strange and dangerous dimension where the rules are different. There, he has to quickly adapt and find a way to use his powers effectively. He might also gain some new powers or abilities in this realm that help him in his fight against the Titan. Along the way, he could learn more about the history of the Titans and the true nature of his own powers.
In some fanfictions, young Harry might kill Voldemort by accident. For example, he could be exploring an old, forgotten chamber in Hogwarts and stumble upon a powerful magical artifact. When he touches it, it releases a burst of magic that targets Voldemort's main soul fragment, wherever he is. This causes Voldemort to perish instantly, leaving Harry to deal with the aftermath and the shock of what he has unknowingly done.
His confidence grows. Killing Titans is no small feat, and each victory makes Percy realize his own strength and capabilities more. He becomes more self - assured in his actions and decisions.
One possible interesting plot could be Naruto and Raven teaming up to face a powerful common enemy. Their different powers from their respective universes, Naruto's ninja skills and Raven's magic, would make for an exciting combination in battle.
One interesting plot could be Percy and his friends going on a quest in a new, undiscovered part of their magical world. Maybe they encounter a rare and powerful magical creature that they have to either befriend or defeat to complete their mission.
One interesting plot could be Percy having to prove himself to Poseidon in a new and unexpected way. For example, he might have to solve a mystery that is directly related to Poseidon's power over the sea but in a realm that is unfamiliar to both of them, like an underwater civilization that has been lost for centuries.
One possible interesting plot could be Percy's struggle with immortality. He might face the loneliness of outliving his friends and loved ones. Another could be how he uses his immortality to protect the world in new and different ways, perhaps by being able to take more risks without the fear of death.
One idea could be Percy having to face a new type of Titan - one that can control time. He'd have to figure out how to break free from the time loops the Titan puts him in. Another could be Percy teaming up with some unlikely allies from the Titan side who have a change of heart and want to stop the more evil Titans. Maybe a Titan who was forced into the fight but doesn't really believe in the cause.
One possible 'Artemis kills Percy' fanfic could be a story where there's a misunderstanding. Maybe Artemis is under a spell that makes her think Percy is a great threat to the natural order she protects, and in the heat of the moment, she takes action. Another could be a darker, alternate universe fanfic where the gods' personalities are twisted, and Artemis has been corrupted into believing Percy is an enemy she must eliminate.
One interesting plot could be Percy and his friends going on a quest in a new, unknown realm. Maybe a place that's not part of the regular Greek mythology but still related in some way.
A great plot might involve Percy and Calypso going on a quest to save a group of demigods. Along the way, they face various monsters and challenges, and their relationship deepens as they rely on each other. They could also discover some hidden secrets about the gods during this quest.