Perhaps Percy gets trapped in a time loop where he has to relive a particular battle as a timeless warrior over and over again until he figures out the key to breaking the loop. In this loop, he slowly uncovers secrets about his past and future that he never knew before.
One idea could be that Percy discovers a hidden power within himself that is related to the concept of a timeless warrior. Maybe he can time - travel in a unique way to different eras of Greek mythology and has to battle ancient evils that are starting to resurface in the modern world.
One idea could be exploring the backstory of the forgotten warrior more in - depth. Maybe they were once a great hero in a forgotten era of the Percy Jackson universe and got lost in time somehow. Another could be that the forgotten warrior has a special power that is now needed to defeat a new, emerging threat. They could be a key to unlocking an ancient prophecy that no one has understood until now.
One idea could be that Percy discovers a hidden power within himself that makes him an even more powerful immortal warrior. Maybe he can control time briefly, allowing him to outmaneuver his enemies in battles. Another could be that he teams up with an ancient immortal warrior from a different mythology, like a Valkyrie from Norse mythology, and they have to combine their powers to defeat a new threat to the world of demigods. And perhaps there's a prophecy that only an immortal Percy can fulfill, which involves traveling to the Underworld and making a deal with Hades to gain an ultimate weapon.
One idea could be Percy having to face a new type of chaos monster that is immune to his usual powers. He would have to find a way to adapt and use his wits more than his strength. Maybe he teams up with an unexpected ally from the realm of chaos.
In a Percy Jackson Warrior of Olympus fanfiction, a great plot could involve Percy being sent into the future of the demigod world. He sees the consequences of the current actions of the gods and demigods. For example, he discovers that a war between the different pantheons has led to the destruction of many demigod camps. So, he has to go back in time and try to change the events that lead to this future. This would involve him having to convince not only his friends but also the gods to change their ways, which would be no easy feat as the gods are often stubborn. He might also meet some of his descendants in the future, and they could help him understand what exactly went wrong.
A powerful Titan who was once a timeless warrior but was banished to the depths of Tartarus could be a great antagonist. He has spent centuries plotting his revenge and now wants to use Percy to break free and destroy the Olympians once and for all.
One idea could be that Percy, as the lone warrior, has to face a new threat alone. Maybe a powerful and ancient monster that has been awakened and no one else believes in its existence at first. He has to rely on his own skills and the limited resources he has.
Since I haven't read this specific fanfiction, a possible plot could be that Percy and Athena are brought together by some ancient prophecy related to the 'timeless warrior'. They start off not really trusting each other due to their different backgrounds. Percy, with his demigod powers, and Athena, with her godly wisdom and powers. As they embark on their journey, they encounter various mythical creatures and obstacles. Along the way, they learn to understand and rely on each other more, and ultimately, they fulfill the prophecy and perhaps bring about a new era of peace or something similar.
One plot idea could be Percy having to team up with Hera to rescue a group of demigods who have been captured by an ancient monster. Hera, despite her usual aloofness, realizes she needs Percy's unique skills.
The plot might involve the forbidden warrior having some special powers or abilities that Percy has to figure out how to work with. It could be that the warrior was once banished or exiled for reasons that are gradually revealed. And Percy, being Percy, gets dragged into the whole situation, and they embark on a journey full of challenges and self - discovery. Along the way, they might encounter other mythical creatures and have to use their combined skills to overcome obstacles.
A plot could be that Sally Jackson plants a lemon tree in her garden that turns out to be a gift from a minor goddess. The lemons from this tree have the power to give those who eat them the ability to see through illusions for a short time. Percy uses these lemons to his advantage when he is on a mission to retrieve a powerful artifact that is guarded by a monster that uses illusions to protect it.