One possible antagonist could be Kronos. Since Percy is becoming a Titan of Time, Kronos, who has his own history with time - related power plays, might see Percy as a threat and try to stop him. Another could be the gods themselves. They might not want a demigod to have such a powerful Titan - like status related to time.
The Fates could be antagonists. They control the threads of time, and if Percy is becoming the Titan of Time, it could disrupt their control. They might try to manipulate events to prevent Percy from achieving this. Also, there could be other Titans who are jealous or afraid of Percy's new - found power. They may view him as an upstart and try to eliminate him. And then there are the creatures of the Underworld, like the demons or some powerful monsters, who could be influenced by the darker forces that don't want time to be in Percy's hands.
Well, perhaps Percy discovers a hidden power within himself that is linked to time. He might start having visions of past and future events and realizes he has the potential to become the Titan of Time. Maybe he has to go on a quest to find an ancient artifact that will complete his transformation.
One possible antagonist could be Orochimaru. He might be interested in Naruto's new Shadowman powers and try to steal them for his own evil purposes.
One plotline could be Percy leading the Titans in a war against the gods. He might use his knowledge of the demigods' weaknesses to plan attacks. Another could be Percy trying to convert other demigods to his Titan side, causing division among them.
Another possible antagonist could be some of the more power - hungry Titans. They might see Percy as a rival or someone who could disrupt their plans. They could try to manipulate him or turn him to their side, and if he refuses, they would become his enemies. Also, there could be some ancient monsters that sense Percy's new power and want to destroy him before he becomes too powerful.
The obvious antagonist could be the Titan of Earth itself. Since it's a powerful being, it could be causing chaos and destruction as it tries to regain its former power and influence. Its very existence might be a threat to the balance of the world that Percy and his friends are trying to protect.
The Titans could be the main antagonists. They might oppose Percy's ascension to Olympian status because they see it as a threat to their own plans of overthrowing the Olympians.
In the fanfiction where Percy Jackson becomes a Titan of Time, his very nature would be transformed. His perception of the world around him would be different as he could now view time as a malleable entity. He might start to see events not just as they happen in the present, but also how they are connected through the past and future. His role as a hero would take on a new dimension. Instead of just reacting to threats as they come, he could proactively shape events. He could potentially save Camp Half - Blood from destruction long before it even became a threat by going back in time and changing certain factors. And in battles, he could create time - loops to trap his enemies or accelerate time to make his attacks faster and more powerful.
There could be other ninja villages that see Naruto's new powers as a threat. They might form alliances to oppose him, fearing that he will use his god - like abilities to dominate them. Also, there might be dark forces or ancient evils that awaken because of Naruto's transformation. These entities could oppose him as they try to maintain the balance of power in their own dark ways.
There could be a rogue legendary Pokemon that sees Ash's transformation as a threat to the natural order. This legendary Pokemon, with its own set of motives, could be trying to prevent Ash from becoming a fully - fledged legendary - like being. It might set up various obstacles and battles for Ash, testing his will and strength at every turn.
The Titans might still be a threat. Even though they were defeated before, they could have some remnants or followers who don't want Percy to become an Olympian because it would strengthen the Olympian side even more.