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.
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 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 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 idea could be Naruto going back in time and preventing the Uchiha massacre. He could use his knowledge of future events to change things for the better in the Uchiha clan, and this would also have a huge impact on Sasuke's character development.
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.
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.
One interesting plot idea could be Percy traveling back in time to the era of the Greek gods' creation. He could witness the birth of the first gods and goddesses and perhaps even play a role in some of the early events. Another idea is him going back to the time of a great hero like Hercules and joining forces with him to complete a quest that was left unfinished. Or he could travel to the future to see what has become of the demigod world and then come back to the present to try and prevent a bad future from happening.
Another plot could be that Hermione goes back in time and ends up changing the relationships between the main characters. Maybe she accidentally causes a rift between Harry and Ron, and then has to figure out how to fix it while also dealing with the overarching threat of Voldemort in that time period.
Well, perhaps a major plot twist is when Percy goes back in time and realizes that the events he thought were accidents in his own time were actually carefully orchestrated by a character from the past. This would add a whole new layer of mystery and conspiracy. Also, he might find that a prophecy he was trying to prevent in his time was actually meant to happen in the past for a greater good, which would be a huge turn for his mission.
A possible plot for an Atreus and Percy fanfic could be centered around a prophecy. The prophecy states that only the combined strength of a Norse and a Greek hero can stop an ancient evil from awakening. Atreus and Percy are brought together by this prophecy. At first, they are skeptical of each other because of their different backgrounds and ways of doing things. However, as they start to face the challenges related to the prophecy, such as deciphering ancient texts and fighting off minions of the evil, they gradually learn to trust and rely on each other. In the end, they succeed in preventing the evil from rising, and their friendship becomes a legend in both their worlds.