Some powerful and greedy mortals who don't care about nature and are trying to exploit it. They might see Percy as an obstacle in their path of building huge factories or mining in protected areas.
There could be a group of dark nature spirits who have been corrupted. They were once protectors of nature but now want to turn the world into a wild and uninhabitable place, seeing Percy as a threat to their new 'order'.
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.
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.
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.
A group of ancient spirits of chaos that have been awakened by the imbalance in natural disasters could be the antagonists. They thrive on destruction and see Percy as a threat to their plans of creating total chaos. They would constantly try to undermine Percy's efforts to control and prevent disasters, and they might possess other powerful beings to fight against him.
Obviously, the God of Death could be a main antagonist if he has some malicious plan. He might be trying to tip the balance of life and death in his favor, causing chaos in the mortal and demigod worlds.
The Titans could be great antagonists. They might see Percy as a threat to their power if he becomes the God of Elements. Kronos, for example, could try to steal Percy's newfound elemental powers to regain his full strength and take over the world again.
It could be dark ninjas who oppose Naruto's new role as the God of Nature. They might want to use the power of nature for their own evil purposes, like creating disasters to gain control over regions.
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.