There might be a group of evil demigods who turned their backs on the good side. They envy Percy's powers and Artemis's status. So, they form an alliance with some underworld creatures and try to take down Percy and Artemis to prove themselves as the most powerful in Oklahoma.
Perhaps a powerful sorcerer who has a grudge against the gods. He sees Percy and Artemis as threats to his plans of ruling over Oklahoma. So, he sends his minions, which are some magically - created beasts, to stop them.
It could be some rogue Titans who escaped from Tartarus. They want to cause chaos in Oklahoma and use Percy and Artemis to their advantage. Maybe they plan to capture them and use their powers to break free the other Titans.
One common antagonist could be an old enemy of Artemis who doesn't approve of her marrying Percy. This enemy might be a powerful Titan who sees their union as a threat to his plans for revenge against the gods.
They might go on a quest to find a lost artifact. Maybe it's a magical item that has the power to control the weather in Oklahoma. They could face various mythical creatures along the way, like a giant snake that guards the artifact.
The Titans could be the main antagonists. Even though they have been defeated before, some of them might have found a way to return or their followers could be causing trouble. They see Artemis as a threat or a target for their revenge against the gods. Another possibility is a new group of powerful monsters that have banded together and are specifically targeting Artemis for her power and influence among the demigods.
The Titans could still be antagonists. Even though Percy is now associated with Artemis, the Titans might see him as a threat or a tool to use against the gods. Kronos, for example, could try to turn Percy to his side by offering him power or by threatening those he cares about in Artemis's group.
In this fanfiction, the Titans could be the main antagonists. They might see Percy as a tool to use against Artemis. Since he's her son, they could try to manipulate him or turn him against her. Another possible antagonist could be the monsters that are drawn to Percy because of his unique heritage. They might sense his power and want to either destroy him or use him for their own evil purposes. Additionally, there could be a group of rogue demigods who are jealous of Percy's connection to Artemis and try to undermine him at every turn.
One possible antagonist could be an old enemy of Percy's who has a vendetta against his son. For example, a powerful Titan who was once defeated by Percy might see the silver son as a way to get revenge. They could try to corrupt the son or use him against Percy and his friends.
The other gods might be antagonists. For example, Zeus could be against Artemis getting Percy back because he doesn't trust Percy or he has his own plans for Percy that don't align with Artemis'. Another god like Ares could also be an antagonist as he might see Percy as a threat and not want him to be on Artemis' side.
Maybe Artemis is on a hunt when she stumbles upon Percy. She might be initially cautious, as she's a goddess and Percy is a demigod. She could question him about his presence in Texas.
The antagonists could be some rogue scientists who are experimenting on demigods like Percy. They might be trying to harness the powers of the demigods for their own gain, whether it's for power, wealth, or some other self - serving reason.
It could be an old enemy from Greek mythology that has been resurrected. For example, a powerful Titan who has a vendetta against the demigods, especially Percy and Annabeth.