One great 'Percy sent back in time' fanfic is 'Back to the Beginning'. It shows Percy having to deal with all the early threats in a new way. Another is 'Time - Twisted Percy' which has an interesting take on how his knowledge of the future changes things. There's also 'Percy's Time - Travel Adventure' where he forms unexpected alliances in the past.
In such a fanfiction, Percy might use his knowledge of the future to change some key events in the past. For example, he could prevent some of the earlier disasters or conflicts in the Percy Jackson universe. Maybe he'd try to stop the first encounter with a certain monster in a different way to avoid the chaos it caused.
It could be an ancient and powerful Titan who was thought to be defeated long ago. Since Percy is in the past, this Titan could be in his prime and see Percy as a threat to his plans of ruling over the world again. For example, Kronos might be even more powerful in this past era and Percy has to find a way to stop him without the help of his modern - day friends.
One common theme is self - discovery. When Percy is sent back in time, he often has to re - evaluate who he is without all the experiences he had in his original timeline. Another theme is the butterfly effect. His actions in the past can have huge consequences for the future. For example, a small decision he makes might change the entire course of a battle.
Obviously, Percy is one of the main characters. But there could be others depending on the specific plot of the fanfiction. Maybe there are some characters from Percy's past that play important roles.
Perhaps Percy will discover secrets about his family or his powers that were previously unknown. Since he's in the past, there could be ancient magic or knowledge that he stumbles upon. This newfound knowledge could give him an edge when he returns to his own time, if he ever does. He might also face challenges like trying to blend in with the people of the past, as his modern ways could make him stand out and cause suspicion.
The plot might involve Percy going back in time and getting involved in a love story that was lost to history. He could fall in love with someone from the past and then have to make a difficult choice when it comes time to return to his own time. It could also be that he has to use his modern knowledge to solve problems in the past that no one else could figure out, which then changes the course of history in small but significant ways.
He could meet his younger self. This would be a really interesting encounter as he could either try to guide his younger self or be shocked at how different he was back then. Another possibility is meeting some of the ancient heroes that he only heard about before. For instance, he might come across Hercules and learn from him.
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.
One character could be a time - keeper or a wizard who has the power to send Percy back in time. This character could be an old and wise figure, full of knowledge about the past and the ways of time travel. Another could be a historical figure that Percy meets in the past. For example, a famous warrior or a philosopher who becomes Percy's ally or mentor.