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.
A group of powerful and ancient spirits that guard the old laws of the gods could be antagonists. They view Percy as a violation of those laws and are determined to erase his existence. Also, there could be a dark sorcerer who has learned of Percy's story and wants to harness his power for his own dark magic. This sorcerer could send all kinds of minions after Percy. And let's not forget the possibility of some of Artemis's own hunters who are against the idea of her having a son and might try to hunt Percy down.