The main antagonist could be Hades in a time loop fanfiction. Hades might be angry that Percy keeps interfering in his plans in the Underworld. So, he uses his power over time in a limited way to create a time loop for Percy. In the loop, Hades sends various underworld creatures to harass Percy, and Percy has to find a way to break the loop and make peace with Hades to stop the cycle.
A really interesting antagonist for a Percy Jackson time loop fanfiction could be a new character, perhaps a powerful magician who is jealous of Percy's powers and status. This magician creates the time loop to drive Percy insane. He constantly manipulates the events within the loop, making it seem like Percy's friends are turning against him. He does this all while hiding in the shadows, making it difficult for Percy to figure out who is really behind his time - trapped misery.
It could be Kronos. Maybe he's found a way to trap Percy in a time loop as part of his plan to stop Percy from fulfilling his destiny. Each time Percy breaks free from the loop a little, Kronos throws new challenges at him.
Another antagonist could be a primordial from another realm who is jealous of the power of the primordial of time. This primordial decides to invade the 'Percy Jackson' world and try to absorb the power of the primordial of time. It would bring a whole new level of threat as it has its own set of powers and minions from its own realm. It could also be that this primordial has the ability to twist time in its own realm, and when it enters the 'Percy Jackson' world, it starts causing chaos with time there as well.
Maybe a corrupted past version of a demigod. For example, a demigod who was promised great power by the Titans and turned against the Olympians. This demigod could see Percy as an obstacle to their plans and try to stop him at every turn. They would know the time period well and could use that to their advantage against Percy.
Well, Percy and Artimis's interaction in a time - loop fanfiction could be really interesting. Percy is a bit of a maverick, and Artimis is a very strict and powerful goddess. In the time loop, Percy would be constantly trying to find solutions and might come up with some unorthodox ideas. Artimis, at first, would be skeptical of these ideas but would gradually see the value in Percy's out - of - the - box thinking. They might have some tense moments when Percy accidentally breaks one of Artimis's rules within the loop. But as they keep repeating the loop and getting closer to the solution, they would develop a deeper understanding of each other. Percy would come to appreciate Artimis's sense of duty and honor, and Artimis would see Percy's loyalty and courage. Their conversations would range from heated debates about the best course of action to more personal exchanges about their hopes and fears within the context of the time loop.
It could be an ancient being that was locked away for centuries but has now been released due to Percy's meddling with time and tides. This being might have a vendetta against the gods in general and sees Percy as a key to achieving its revenge. It could have the power to manipulate time in a different way than Percy, causing all sorts of chaos and destruction. For example, it could create time loops that trap people in never - ending cycles of pain and suffering, and Percy has to figure out how to break these loops and stop the being.
One possible plot could be Percy being stuck in a time loop where he has to keep saving Artimis from a recurring threat. Maybe a powerful Titan is constantly finding new ways to capture her, and Percy has to figure out the pattern within the loop to stop it for good. Another idea is that the time loop is a result of a curse placed on Artimis, and Percy has to team up with the hunters of Artimis to break the curse before the loop consumes them all.
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.
It could be a powerful demigod who has been corrupted by power. This demigod wants to be the sole champion of the arena and will stop at nothing to defeat Percy.
A powerful demigod who has been corrupted by the idea of primordial power could be the main antagonist. They might believe that by allying with a primordial or obtaining their power, they can become greater than the gods. This demigod would then oppose Percy, who stands for the protection of the current order.
A powerful Amazonian sorceress who has turned against her own kind and is now trying to use dark magic to take over the world. She could see Percy as an obstacle in her path and constantly try to defeat him.
Maybe a rogue demigod who blames Percy for something and wants to make his exile permanent. This demigod could have some dark powers or be in league with some evil forces. They would constantly try to thwart Percy's attempts to return or find a way to clear his name.