Some of the powerful demigods could be antagonists too. They might be jealous of Percy and his relationship with Artemis. They could try to stop Artemis from getting Percy back by creating obstacles or spreading false rumors about Percy. Maybe they want to be the ones to go on quests with Artemis instead of Percy.
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.
One possibility is that Artemis realizes Percy's unique skills and abilities are crucial for a quest she has in mind. Maybe there's a powerful enemy emerging that only Percy can help defeat. So, she tries to convince Percy to join her side again, perhaps offering him some kind of reward or appealing to his sense of heroism.
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.
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.
Some members of the ANBU themselves could be antagonists. They might think that Naruto belongs with them and that Kushina is interfering in their affairs. They could try to keep Naruto in the ANBU by any means necessary, including using force or setting up traps for Kushina when she tries to approach Naruto.
Maybe a new group of rogue ninjas who see Naruto as a threat to their plans. They've kidnapped him and are holding him captive. Anko would then have to fight these ninjas who are well - trained and have their own unique abilities. They might be after Naruto's Nine - Tails chakra for some evil purpose.
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.
A strict teacher could be an antagonist. Maybe this teacher has old - fashioned views and doesn't like Percy's unorthodox ways or Artemis's independent streak. The teacher could constantly give them detention or try to stop them from participating in certain school events that are important for the plot.
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.