Perhaps in the fanfiction, the Titan of Earth has a connection to Percy's family heritage that he was unaware of. This discovery leads Percy on a journey to understand his own powers better. He has to face challenges like dealing with the Titan's minions who are trying to use Percy to fully revive their master. Along the way, he also has to balance his normal demigod life and relationships.
One idea could be Percy having to face an ancient sea curse that was long forgotten. Maybe it was set by a powerful Titan before his time and it starts causing strange phenomena in the ocean, like ships disappearing without a trace. He has to team up with some new sea creatures he's never encountered before to solve it.
Perhaps Percy, as the God of the Earth, has to plant the seeds of a new magical forest to restore balance to the world. But an evil force is trying to stop him by sending monsters to attack him and the forest he is trying to create. He has to use his earth powers to defend and also enlist the help of nature spirits like dryads. He also discovers that this new forest has a connection to an ancient prophecy that could change the fate of the world.
One interesting plot idea could be exploring the untold stories of the minor characters in the 'Titan's Curse'. For example, what the satyrs were up to when they weren't with Percy and his friends.
One idea could be Percy discovering new powers inherited from his Titan heritage that set him apart from other demigods. For example, he might be able to control time on a small scale, like slowing it down during a battle to gain an advantage.
One idea could be that Percy has to hide his Titan heritage from the other demigods at Camp Half - Blood. He discovers new powers related to his Titan side that are different from his godly powers. For example, he might have the ability to control certain aspects of nature in a more primal way compared to his Poseidon - related powers.
One plot idea could be Percy having to team up with Hera to rescue a group of demigods who have been captured by an ancient monster. Hera, despite her usual aloofness, realizes she needs Percy's unique skills.
In this fanfiction, imagine if Percy Jackson was transported to the Naruto universe during a battle with the Titans. He could use his demigod abilities to help Naruto and his friends against a new threat that has some connection to the Titans. Maybe the Titans' power is being used to create a super - villain in the Naruto world.
The obvious antagonist could be the Titan of Earth itself. Since it's a powerful being, it could be causing chaos and destruction as it tries to regain its former power and influence. Its very existence might be a threat to the balance of the world that Percy and his friends are trying to protect.
One idea could be Percy has to face Medusa again, but this time she has an army of stone minions. Another could be that Medusa was actually cursed by a more powerful god and Percy has to find a way to break it. Maybe Percy discovers Medusa has a long - lost relative who is being held hostage and he has to make a deal with her.
Well, a cool plot might be that there's a merman who has a special power to control water currents, and Percy needs his help to find a lost magical artifact deep in the ocean. In return, Percy has to help the merman get accepted by the surface - dwelling gods. Or perhaps a merman is wrongly accused of stealing Poseidon's trident and Percy has to prove his innocence.