In this fanfic, Percy could be driven to revenge because of a personal loss caused by the Olympians. He might start by infiltrating Olympus under a false identity. Once inside, he begins to subtly undermine the gods' authority. He could spread rumors among the minor gods and spirits, causing unrest in the divine realm. As the chaos builds, Percy's revenge plot unfolds, leading to a big showdown between him and the Olympians.
Annabeth could be his ally. She's always been by Percy's side and if he has a just cause for revenge against the Olympians, she would likely support him. Her strategic mind would be a great asset in planning against the gods.
One idea could be that Percy discovers the true identity of the person who wronged him and plans an elaborate ruse to publicly expose them. This would not only get revenge but also ruin their reputation.
It could be an old rival from Percy's past, someone who has always been jealous of Percy's abilities and has finally found a way to cause him great harm. This person would be a perfect antagonist as Percy would have a strong motivation to get revenge on them.
One idea could be Percy having to face a new and powerful monster that is a combination of different mythological creatures. For example, a chimera with the head of a hydra. This would create a unique and challenging battle for him.
Percy Jackson, the son of Poseidon, realizes he's a demigod. Then he gets thrown into a world full of danger. There are quests he has to undertake. For example, he has to retrieve Zeus' lightning bolt to prevent a war among the gods. Along the way, he meets other demigods and has to face many monsters. His friendship with Annabeth and Grover also evolves as they face these challenges together.
A great plot could involve Percy being a character in a magical world. He's been on a long and dangerous adventure and has finally returned home. All he desires is to cuddle with his family or his beloved pet. The fanfic could then focus on the tender moments of their reunion, with lots of hugs and cuddles, and Percy finally feeling safe and loved in their embrace.
The plot could involve Percy being bullied by a new group of monsters in disguise at Camp Half - Blood. These monsters, looking like normal campers, could be trying to weaken Percy before a big battle. Percy would start to doubt himself because of their constant taunts and pranks.
The plot could also involve Thor being curious about the demigod world. He might have heard tales of Percy's adventures and wants to meet him to learn more about how demigods operate, their powers, and their training. It could be a kind of cultural exchange between the Norse and Greek mythological worlds.
One interesting storyline could be Artemis teaming up with Percy to face a new and powerful threat that the Olympians are unaware of. It could involve them traveling to a forgotten part of the mythical world.
Annabeth is also a very popular choice. Her intelligence and her relationship with Percy are often explored in fanfics. Writers might create scenarios where she uses her smarts to solve complex problems or where her relationship with Percy is put to the test in new and interesting ways.