Here's a plot. The gods, fearing a future threat that they've glimpsed, send Percy on a time - travel journey. He travels to different periods in Greek mythology, like the time of the Trojan War. There, he meets heroes like Achilles and Hector. He has to figure out how the events of that time are related to the future threat. Along the way, he discovers secrets about the gods' past actions and how they might be repeating the same mistakes. This could involve him interacting with different versions of the gods at different times, some more benevolent and some more cruel than he's used to.
Well, a cool plot could be Percy accidentally time - traveling during a battle with a powerful enemy. He ends up in a time when the gods were more directly involved with mortals. He has to find a way to get back to his own time while also dealing with the fact that his presence in the past is changing the future. Maybe he meets a young version of one of his parents and has to make sure not to disrupt their future relationship. Also, the gods could be using him as a pawn in their own time - related schemes.
One interesting plot idea could be Percy traveling back in time to the era when the gods first came to power. He could witness the creation of the Olympian council and get involved in the early power struggles among the gods. Another idea is that a god sends Percy on a time - travel mission to fix a mistake in the past that is now threatening the present world of demigods. For example, if a powerful magical artifact was lost in the past and its absence is causing chaos in the present.
One idea could be Percy using his new powers to travel through different time periods in the demigod world and fix historical disasters. For example, he could go back to the time of the Titan War and prevent certain events that led to great losses. Another plot could involve him exploring different dimensions in space, finding new and strange mythical creatures that exist only in those far - off realms. He might also have to deal with the consequences of messing with time, like creating alternate timelines that start to collide with the original one.
One idea could be Percy discovers a new power related to controlling time and tides that was hidden from him all along. Maybe he can freeze time during a battle to gain an upper hand against his enemies. Another plot could involve Percy traveling through different time periods using his newfound god - like abilities to correct past mistakes in the world of the gods.
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.
One idea could be Percy having to use his new powers of light to battle against an ancient darkness that is threatening to engulf the world. He might have to journey to the underworld to find the source of this darkness and use his light to purify 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.
One plot idea could be Percy using his time - controlling powers to go back in time to prevent a great disaster from happening in the past that would have otherwise led to the destruction of a civilization. For example, he could stop an ancient evil sorcerer from casting a spell that would have wiped out an entire race.
Perhaps in the fanfiction, the Elder Gods start to interfere with the modern world of demigods more directly. Percy and his friends notice strange phenomena, like the weather going haywire and magic behaving erratically. They soon find out it's because an Elder God is trying to regain its former power and is using the chaos to break through the barriers that keep it in a weakened state. Percy then has to find a way to stop it by using his unique connection to water and the help of his father, Poseidon.
One idea could be Percy having to face a new type of demon that is immune to water, his usual weapon. So he has to find a new way to fight. Another could be Percy being tricked into serving the God of Demons for a while and then realizing his mistake and trying to break free.
One idea could be exploring a new adventure where Percy has to team up with a minor god that has been forgotten. They could go on a quest to regain the forgotten god's power and status in the pantheon.
Perhaps Percy, as the God of Wisdom, has to deal with a situation where his wisdom is being questioned by the other gods. They don't believe that a former hero like him can handle the responsibilities of wisdom. So he has to prove himself by solving a complex problem that has been plaguing Olympus for centuries. He uses his unique perspective as a demigod - turned - god to come up with a solution that no one else could think of.