In some fanfictions, it could be an ancient and powerful Titan who has escaped from Tartarus and is seeking revenge on the demigods. This Titan might see Percy and Thalia as the main obstacles in his path to domination.
A new and mysterious god or goddess who has emerged from the shadows. This deity might have a different view on how the demigod world should be run and sees Percy and Thalia as the ones who are upholding the old, 'wrong' order. So they send all kinds of trials and enemies their way to break them down.
It could be the agents of Order who see Percy and Thalia joining Chaos as a threat to the balance they strive to maintain. They might send their best warriors to stop Percy and Thalia from carrying out Chaos' plans.
The main characters are Percy and Thalia. Percy is often known for his bravery and quick thinking in various situations. Thalia, on the other hand, has her own set of unique abilities and a strong personality. In the context of the 'Crying Guardian' fanfiction, they both take on the role of protectors with a special connection related to their crying, which might be a source of power or a symbol of their deep emotional bond.
Maybe some of the more vengeful gods could be antagonists. For example, if Hera doesn't like Percy's decision to adopt Thalia, she could send various challenges their way. Also, there could be some ancient monsters that are awakened by Thalia's presence, and they would want to destroy her. These monsters could be sent by some minor gods who are jealous of the attention Thalia might get from the major gods.
It could be some of the jealous gods. Since Percy, Thalia, and Nico are living on Olympus, which is quite an unusual thing for demigods, some gods might not like their presence and try to get rid of them. For example, a god who feels that their power is being threatened by these demigods.
The Titans could be great antagonists. They might see the Hunt as a threat to their plans for power and send their minions to disrupt it. Percy would have to fight off these minions, like the Telkhines, who are known for their metal - working skills and could create powerful weapons against the Hunt.
It could be an old enemy of Percy's, like a vengeful god or a powerful monster. They might be targeting Percy's guardian to get back at Percy.
The Titans could be the main antagonists. They might see Percy becoming a guardian of the Hunt as a threat to their plans. Since the Hunt has its own power and influence, the Titans would want to disrupt it. They could send their minions to attack the Hunt, and Percy would have to use his skills and newfound position to fend them off.
One great 'Percy Jackson Guardian Thalia Fanfiction' is 'Thalia's New Quest'. It shows Thalia in a whole new adventure, facing challenges that really test her skills as a guardian. The author does a great job of portraying her strong and independent nature.
The fanfiction could revolve around the relationship between Percy and Thalia as they work to safeguard Percy's guardian. It could show how their friendship or partnership develops during the course of their adventure. For example, they could start off not getting along so well, but as they face challenges, they learn to trust and rely on each other more.
Another possibility is a group or organization that has different ideals. If Thalia and Luke are part of a group that values something different from them, that group could oppose their decision to have a child and become the main antagonists, trying to stop the pregnancy or take the baby away once it's born.