A new main antagonist could be a forgotten or banished god. There are many minor gods in Greek and Roman mythology that haven't been explored much. One of them could be angry at the demigods for getting all the attention and decide to cause chaos. For example, a god of misfortune who decides to curse the demigods with a series of bad luck that leads to dangerous situations.
There could also be some ancient monsters or spirits that are tied to Olympus' existence. They might rise up to defend it because their own survival is linked to the continued existence of Olympus. These could be powerful beings that Percy has never faced before, adding an extra layer of danger to his quest to destroy Olympus.
The Olympians themselves could be the main antagonists in a sense. Since Percy is betraying them, they would be the ones trying to stop him. Zeus, with his power and pride, would likely be at the forefront, seeing Percy's betrayal as a direct threat to his authority.
One possible plot development could be Percy and his friends facing a new, even more powerful enemy. Maybe an ancient primordial being that was previously sealed away has been awakened. For example, a new Titan or a primordial chaos entity. Another development could be Percy exploring his new powers or abilities that he might have gained after the events of Heroes of Olympus. He could be training other demigods as well, passing on his knowledge and skills.
The spirits of the maze itself could turn against Percy. They could be angry at all the demigods who have been trespassing for years and decide to stop Percy from getting through by any means necessary.
A lesser - known but really interesting fanfiction is 'Annabeth's Quest for Wisdom'. This story follows Annabeth as she goes on a solo quest to gain more knowledge and wisdom. It shows a different side of her character, her determination and her thirst for knowledge in a very engaging way.
The main characters include Percy Jackson, who is a well - known demigod. Then there's Jason Grace, a Roman demigod with his own set of powers and challenges. Piper McLean, who has special abilities and plays an important role in the group's adventures. Annabeth Chase, Percy's girlfriend, is also a significant character. She is smart and brave, often helping the group outwit their enemies with her strategic thinking.
The Giants might also be antagonists. They could band together and target Percy's kingdom, believing that by destroying him and his rule, they can undermine the power of the gods further. They might use their brute strength and strategic thinking to attack Percy from different fronts.
Another option could be a group of rogue demigods who have turned to the dark side. They follow Percy to Forks, hoping to find some powerful artifacts there. They don't care about the vampires or werewolves and are willing to cause destruction to get what they want. Percy has to stop them while also dealing with the local supernatural creatures.
A half - human, half - nymph character could be interesting. This character could have unique powers related to nature that would complement Percy's water - based powers. They could also bring a fresh perspective on the relationship between humans and nature spirits in the world of Greek mythology.
It could be a new demigod who has a grudge against the Olympians. This demigod might be using Percy as a tool to carry out their assassination plans.