One possibility is that Kronos captures Percy to steal his powers. Percy, being a powerful demigod, has abilities that Kronos desires for his own evil plans. While captured, Percy may discover some secrets about Kronos' past that could be used against him. He has to use his wits and any small resources he has in the prison to outsmart Kronos and break free.
His friends like Annabeth might come to his rescue. Annabeth is smart and resourceful, she could figure out where Percy is being held and lead a rescue mission.
One possible plot could be that Percy discovers his true relation to Kronos early on. This revelation could cause a great deal of inner turmoil as he has to come to terms with being related to the main villain. He might then struggle with whether to side with his friends or his new - found brother, creating a complex moral dilemma.
One possible plot could be Percy starting off reluctantly working for Kronos. Maybe Kronos has some sort of leverage over Percy, like threatening his friends or family. Percy then has to play a double - agent role, secretly trying to undermine Kronos' plans while appearing to be loyal. He could gather information about Kronos' forces and weaknesses and pass it on to the good guys.
The plot could involve her on a quest. Maybe she's searching for a way to break the cycle of Kronos' evil. She might team up with Percy Jackson and other characters. They could face various monsters and challenges along the way. And she could be constantly battling her own inner demons related to her father's nature, like a tendency towards power - hunger that she has to overcome.
One possible plot could be Percy's journey to discover his true identity as the son of Kronos and Rhea. He might face challenges from the other gods who fear his power. For example, Zeus could try to stop him from fully realizing his potential. Percy would also have to deal with his own internal struggle. He grew up believing he was Poseidon's son, so this new revelation would shake his world. He could start a quest to find out more about his parents' relationship and what it means for him in the grand scheme of the mythological world.
The plot could start with Percy being somehow influenced by Kronos, maybe through a dream or a cursed object. Then he slowly starts to act in ways that are out of character for him. He might begin to gather followers who are also attracted to the power that Kronos represents. Eventually, he could lead an uprising against the gods, believing that he can create a new world order under Kronos's ideology.
Well, one possible plot could be Percy having to hide his relation to Kronos while still fighting against the Titans. He might struggle with the internal conflict of having a connection to such an evil entity as Kronos while trying to protect his friends at Camp Half - Blood.
Well, perhaps Percy discovers a way to revive Kronos out of desperation to right some past wrongs in the new age. Maybe he thinks Kronos can help against a new, greater threat.
A plot could be that Kronos was once a good god but was corrupted by an ancient evil force. Through a series of events in the fanfiction, Percy and his gang find a way to cleanse Kronos of this corruption. Kronos then, in his redeemed state, helps them fight against the remaining threats to the world of demigods. He could use his knowledge of the old times and his powers to aid in battles and also offer wisdom to the young heroes.
One possible plot could be Percy's discovery of his true parentage as the son of Kronos and Rhea. This could lead to a huge power struggle within the demigod world. Other gods and demigods might fear his potential power and try to either eliminate him or recruit him to their side.