Perhaps the Staff of Fire gets stolen and Percy has to go on a quest to find it. Along the way, he teams up with some new demigods he's never met before. They face various challenges like a maze full of fire - breathing monsters. Percy discovers that the Staff has a hidden power that can only be unlocked by a certain combination of his own powers and those of his new friends.
A dark demigod who was once rejected by the gods and now wants to use the Staff of Fire to take revenge. He could be really powerful and have the ability to control shadows, using them to his advantage against Percy and his friends.
One idea could be that Percy discovers a hidden power within the Staff of Hermes that can open portals to different mythological realms not yet explored in the original series. He could then embark on a journey to these new places, facing new monsters and making unexpected alliances.
One idea could be Percy Jackson teaming up with Kratos from God of War. They could face a common enemy that threatens both the Greek and Norse worlds. Maybe a powerful Titan who has found a way to break free from Tartarus and is causing chaos everywhere.
One idea could be Percy having to face a new and powerful sea monster that is immune to his normal powers. He has to rely on his wits and the help of some unexpected allies from the Roman side. Another could be Percy getting lost in an underwater maze that was created by an ancient sea god as a test for him. Maybe he discovers a long - lost relative who is also a demigod and they team up to solve a mystery that threatens both the Greek and Roman camps.
One idea could be that Percy and Grover discover a hidden part of the Camp Half - Blood forest that has magical creatures not known before. They have to figure out how to protect these creatures from a new threat.
One idea could be Percy discovers a new power related to fire that was dormant within him. Maybe it was a gift from an ancient and forgotten god. He then has to learn to control it without the help of his usual mentors like Chiron.
One interesting plot twist could be that the Staff of Olympus has a hidden power that no one expected. For example, it might be able to reverse time in a small area, which changes the whole dynamic of a battle. Another could be that a character who was thought to be an enemy turns out to be a key ally in the quest related to the Staff. Maybe they were under a spell or had a secret mission all along.
One plot idea could be Percy having to team up with Hera to rescue a group of demigods who have been captured by an ancient monster. Hera, despite her usual aloofness, realizes she needs Percy's unique skills.
One idea could be Percy discovers a long - lost Hephaestus - made weapon that has special powers. It could be a sword that can cut through any material, but it has a mind of its own and Percy has to figure out how to control it while also dealing with other monsters that are after the weapon.
In a fanfic, perhaps Percy and his friends accidentally stumble upon Medusa's lair while on a quest. But instead of immediately attacking, Medusa offers them a deal. She says she will give them a valuable clue to complete their quest if they help her get revenge on one of the minor gods who wronged her in the past. Percy and his friends then have to decide whether to trust Medusa or not.