Perhaps a powerful and corrupted demigod could be the main antagonist. This demigod might be jealous of Artemis' power and popularity among the other demigods. He devises a plan to harm her, and Percy, being loyal to Artemis, has to give his life to save her from this rogue demigod.
A group of dark magic - using monsters could be the antagonists. They band together with the sole purpose of destroying Artemis. Percy, who is always in the thick of things, tries to fight them off but realizes that the only way to truly stop them is by sacrificing himself, thus becoming the hero who dies for Artemis.
It could be a powerful Titan who has escaped from Tartarus and blames Artemis for something in the past. So he's out to get her, and Percy has to stop him at all costs, even if it means his death.
Maybe the main antagonist is an ancient evil that has awakened. It has the power to manipulate Artemis's mind, making her kidnap Percy against her will. This evil entity wants to use Percy as a sacrifice to gain more power. So, while Artemis is the one who physically kidnaps Percy, the real force behind it, the ancient evil, is the true antagonist.
It could be an old enemy from Percy's past life who has also found a way to be reincarnated. This enemy might hold a grudge against Percy for something that happened in their previous encounter and is now seeking revenge in this new life.
A powerful Titan who has escaped from Tartarus. This Titan blames Percy for his previous imprisonment and wants to make Artemis suffer as well for her role in keeping the Titans locked away. He uses Percy's death as a way to manipulate the situation and turn the demigods against Artemis, or he might try to steal Percy's powers that were left behind after his death for his own evil purposes.
The Titans could also serve as antagonists. If they sense that the union of Percy and Artemis could strengthen the power of the gods, they would do everything they can to stop the marriage. They could send their minions to attack Percy and Artemis, or they could try to corrupt one of them, perhaps by offering Percy great power in exchange for abandoning Artemis, or by making Artemis believe that Percy will bring destruction to her followers if she marries him.
A powerful Titan who has escaped from Tartarus could be the main antagonist. He might be seeking revenge on Artemis for her role in the previous battles against the Titans. This Titan could have new and terrifying powers that make him a formidable foe for Percy and Artemis.
A powerful Titan who has a grudge against both Percy and the Hunters of Artemis could be a great antagonist. Maybe he wants to use the power of the Hunters for his own evil purposes or destroy Percy to get revenge on the gods.
There could also be a group of rogue demigods who oppose the daughter of Artemis. They might be jealous of her powers or her connection to Artemis. They could try to sabotage her and Percy's quests and cause trouble for them at every turn.
It could be an ancient and powerful Titan that has risen again. This Titan might have a vendetta against Poseidon and targets Percy as a way to hurt him. Since Percy is Poseidon's son, his death in Poseidon's arms would be a great blow to the sea god.
It could be a new, powerful Titan that was previously unknown. This Titan might have a vendetta against Percy specifically, seeing him as a major threat to their plans. Maybe they manage to kill Percy, but his rebirth sets in motion a series of events that will lead to their ultimate downfall.