One idea could be Percy going back in time to the era of the first Olympian gods and having to deal with the raw, unbridled power of those early deities. He might find himself in the middle of a power struggle between Zeus and his father, Kronos, before the Titanomachy. Another plot could involve Percy trying to prevent a major disaster in the past that has far - reaching consequences in his own time. For example, stopping the release of a powerful monster that was supposed to be sealed away long ago.
A possible plot is that Percy is sent back to the time right before the creation of Camp Half - Blood. He could then play a role in its foundation. Maybe he meets the very first demigods who had to fend for themselves without a proper safe haven. He could introduce some of the modern ideas he has about training and protecting demigods, which would change the course of how Camp Half - Blood developed over time.
One idea could be Percy going back in time to the era of the Greek gods' creation. He could witness the birth of the first demigods and learn the true origins of his powers. Another plot could involve him traveling back to a time when a great prophecy was first made and trying to change the course of events to prevent a future disaster.
One interesting plot idea could be Percy going back in time to the era of the Greek heroes. He could meet Hercules and learn from him. Maybe he would have to face the same challenges Hercules did, but with his own modern - day skills and knowledge. Another idea is that he goes back to the time when the gods first emerged and gets involved in the power struggles among them. This could change the entire course of mythology as we know it.
Maybe Percy travels back to the founding of Camp Half - Blood. He could meet the first demigods and see how the camp was established. He could also learn ancient fighting techniques and bring them back to his time to use against modern enemies. This would add a really cool element of anachronism to the story.
One idea could be Percy discovers a new power related to time during a quest. For example, he might be able to freeze time briefly to escape a dangerous enemy. Another could be that he travels back in time to change an event in the past that has a major impact on the present in the world of the Gods.
One idea could be Percy reliving the day he discovers his true identity over and over. Each time he tries different ways to break the news to his mom, like one time being super casual, another time making it a big dramatic reveal. Another plot could be him reliving a battle, and each loop he figures out a different strategy to defeat the monsters more efficiently.
One idea could be Percy having to time - travel back to the very beginning when the primordials first emerged. He could accidentally disrupt the balance of power among them, leading to all kinds of chaos in the present. For example, if he stops a primordial from creating a certain element that is crucial for the modern world's magic. Another plot could involve Percy teaming up with a primordial of time to fix time - loops that are causing disasters in different eras. Maybe he has to enter the time - realm of the primordial, which is a completely different and dangerous place filled with temporal anomalies.
One idea could be that Percy goes back to the time when the gods first came to power. He could try to change the way the gods interact with demigods from the start, leading to a whole new world order for demigods. Maybe he'd teach the early demigods some of the skills he has, like advanced sword fighting techniques.
In a 'Percy back in time fanfic', Percy could travel back to the time of the Titans' rule. He would be in a world full of chaos and oppression. Maybe he teams up with some of the early heroes who were fighting against the Titans secretly. He could learn new powers or fighting techniques from that era. His presence there could also cause some of the Titans to be more cautious, which could potentially change the outcome of the Titan - god war. This would then have a huge impact on the present that he came from, as the world would be very different in terms of power dynamics and relationships between different mythological beings.
If Percy is thrown back in time, his powers could be enhanced. In the past, magic and the power of the gods were more prevalent. The very air might be charged with more divine energy. His ability to communicate with sea creatures could expand, as there could be species that are now extinct or have evolved in his own time. However, he might also face limitations. For example, if he goes back to a time when the gods were at war, some of his powers related to Poseidon might be restricted depending on his father's situation in that war.
In a Percy Jackson time travel harem fanfiction, Percy could time - travel to a time when the Titans were still ruling. He befriends some female Titans who are intrigued by his modern - day demigod ways. Along with some female demigods from different points in history that he also meets during his travels, they form an unlikely group. Their main goal could be to stop a future threat that Percy knows about from his own time, but they also have to deal with the complex relationships within the harem.