In this kind of fanfiction, the Titans could also be antagonists. They might want to use Percy's power over time to their advantage. Maybe they think they can rewrite history in a way that would bring them back to power. Kronos, in particular, if he were somehow still a threat, could be a major adversary. He could try to manipulate Percy into using his time powers in a way that would benefit the Titans, like going back and changing events that led to their downfall.
There could be a group of time - based monsters or creatures. These could be entities that exist outside of normal time and they see Percy's new role as an interference. They might try to attack Percy and his friends to stop him from having too much influence over time. They could be like time - wraiths, beings that feed on the chaos of disrupted timelines. Their goal would be to keep time in a state of disorder and Percy's attempts to control it would be a direct threat to them.
The old - fashioned and conservative gods who don't like the idea of a new, young god like Percy shaking up their established order could be antagonists. They might see him as a threat to their power and try to undermine him.
The Olympian gods who feel threatened by Percy's secret god - like status could be the main antagonists. They might see him as a threat to their power and order.
One possible antagonist could be Kronos again. Since Percy now has time - related powers, Kronos might see this as an opportunity to use Percy to his advantage. He could try to manipulate Percy into changing the past in a way that would free him from his eternal prison. Another could be a new, unknown deity who is jealous of Percy's newfound power over time and space and wants to take it for himself.
Perhaps some of the older, more traditional gods could be antagonists. They might see Percy as a threat because he's now the God of Dinosaurs and they fear the power he could gain from controlling these ancient and powerful creatures. For example, Zeus might be worried that Percy's new role could disrupt the existing power balance among the gods.
The Titans could be the main antagonists. They might want to use the powerful weapons Percy is associated with to regain their power and overthrow the Olympians.
The Titans could still be antagonists. Even though they were mostly defeated in the original series, some of them might have found a way to use the power of the night to their advantage. For example, Kronos could have some remnant of his power that is now tied to the night and he wants to use Percy, as the God of Night, to regain his full strength.
The main antagonists could be the Titans who are still bitter about their defeat and want to disrupt the new balance that Percy is trying to create. They might see Percy as a threat to their plans of regaining power. Another group could be some rogue demigods who have been corrupted by power and don't want Percy to enforce balance as it would limit their own abilities to do as they please.
The Titans could be the main antagonists. They might see Percy as a threat to their plans of regaining power. Since Percy has the power of the stars, they could try to steal his powers or stop him from using them to protect the world.
The Titans related to chaos and destruction of nature could be great antagonists. For example, Cronus might want to use Percy's newfound power as the God of Nature to further his own dark plans.
A group of powerful and corrupted demigods could also serve as antagonists. They might have been promised great power by some dark force if they can disrupt the work of the Fates. So they set out to target Percy and try to stop him from fulfilling his role as the God of Fates.