A powerful and corrupted demigod who was once rejected by the Hunters could be the main antagonist. This demigod has since found a way to gain dark powers and is now determined to make the Hunters pay for their rejection. They start by targeting the Hunters' camps and stealing their magical weapons, forcing Percy to step in and stop them.
It could be an ancient and vengeful Titan who was once imprisoned by the Hunters' predecessors. Now free, it wants to destroy the Hunters to regain its lost power and take revenge on those who once locked it away.
Maybe a group of rogue monsters led by a super - intelligent Cyclops. The Cyclops has a grudge against Percy specifically because Percy has defeated so many of its kind in the past. It gathers other powerful monsters like harpies and hellhounds, and they form a plan to ambush the Hunters when Percy is around, hoping to take them all out at once.
Artemis herself could be considered the 'Guardian of the Hunters' in a way. Since she is the goddess who leads the Hunters, she has a natural role of protecting them. But in fanfiction, it could also be a character like Thalia. She has a strong connection to the Hunters and her skills and powers would make her a great guardian.
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 possible main antagonist could be a rogue demigod who was once rejected by the Hunt and now wants revenge. This demigod could have allied with some powerful monsters and is constantly causing trouble for the Hunt. For example, they might be stealing important artifacts or sabotaging the Hunt's missions.
A powerful Titan who was banished to the moon in the past and now wants revenge on the gods by destroying the moon could be the main antagonist.
It could be a powerful demigod who has been corrupted by greed. He wants to use the power of the earth for his own gain, like creating his own kingdom or something. So he's trying to stop Percy from protecting the earth because Percy stands in his way.
A good antagonist could be a powerful and vengeful centaur. Centaurs are known for their knowledge and combat skills. This centaur could have a grudge against both Percy, perhaps because he defeated some of the centaur's friends in the past, and the Hunters of Artemis because they protect the forests that the centaur wants to claim for his own evil purposes.
It could be a rogue demigod who wants the throwing stars for their power. This demigod might have been shunned by the other demigods and sees the stars as a way to gain revenge and power over them.
A great main antagonist could be a powerful Titan who has somehow escaped from Tartarus and is seeking revenge on the gods. He could target the Hunters of Artemis because they are loyal to Artemis, a goddess he has a grudge against, and Percy because of his connection to Poseidon.
A powerful and corrupted nymph could be the main antagonist. She was once a friend to the Hunt but was lured to the dark side by some ancient magic. Now she wants to destroy the Hunt and take over their territory. She uses her powers over nature to create all sorts of obstacles for the Hunt and Percy, if he's involved. She could also be very manipulative, turning some of the Hunt members against each other.
In Percy Jackson fanfiction, the 'Guardian of the Hunters' could be a character who protects the Hunters. It might be someone with special powers or a deep connection to the group. Maybe it's a new character created by the fanfic writer to add more drama and adventure to the story.