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.
Obviously Percy would be one of the main characters. There might also be other characters from the Percy Jackson series like Annabeth. Maybe the gods of Olympus would also be important characters as they are the ones whose home, Olympus, is being destroyed.
I'm not entirely sure specifically as it's a fanfic. But generally, it might involve Percy having some extraordinary power or facing a situation that forces him to destroy Olympus. It could also be a really creative take on the existing Percy Jackson universe, where the author wants to explore a different and more radical storyline compared to the original.
Grover could also be a major character. As Percy's best friend, he would probably be involved in trying to stop Percy if he thinks Percy has gone too far. Or, if he sees the corruption in Olympus, he may side with Percy. Grover's connection to nature means he could play an important role in the events that unfold, like using nature spirits to either protect Olympus or aid Percy depending on his stance.
One possible plotline could be Percy being betrayed by the gods and thus deciding to take on Olympus. Maybe he discovers some dark secrets about the gods' rule that make him believe destruction is the only way to bring true justice.
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.
The Titans could still be antagonists. In the Percy Jackson universe, they are powerful and malevolent. A princess in a fanfic might have to face their wrath as they try to overthrow the gods and take over the world. They would see the princess as a threat, especially if she is allied with the gods or has some special power that could stop them.
A new and powerful Titan who has been hiding and gathering strength all this time. He could be jealous of the Olympians' power and wants to take over. Percy would have to figure out his weaknesses and stop him before he causes too much destruction.
Well, it depends on what you like. If you're a big fan of the Percy Jackson series, it might be interesting as it often takes the beloved characters and the world of Olympus in new and creative directions. Some fanfics in this vein are really well - written and add depth to the existing lore.
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.
It's possible that the main antagonist is a new mythological creature that has the ability to manipulate minds. It has lured Percy and other demigods to the mental hospital and is slowly driving them insane in order to feed off their fear and chaos energy.