Well, a cool plot might be that Percy somehow gets connected to an ancient power source that gives him the power to create portals. He could use these portals to quickly travel between different mythological realms, but he also has to be careful not to let the wrong creatures or powers through. Or perhaps he gains the power of elemental manipulation related to water, not just controlling it but also changing its state and properties. For example, he could turn water into ice strong enough to build impenetrable fortresses. And then there's the idea of Percy getting a power that lets him see the true forms of all magical beings, which would be both a blessing and a curse as some forms are too terrifying to behold.
In a fanfiction about Percy Jackson having more power, he could be given the power of prophecy. But it's not as straightforward as it seems. The prophecies he receives are all jumbled and he has to figure out how to decipher them. This new power would put him in a difficult position as he has to make decisions based on these unclear prophecies. Also, he could gain the power to heal any injury, not just physical but also mental and emotional. This would make him a target for those who want to use his power for their own gain. And what if he gets a power that allows him to share his powers with his friends for a short time? This would lead to some really interesting team - up scenarios and battles.
One idea could be Percy discovers a new power related to time during a quest. For example, he might be able to freeze time briefly to escape a dangerous enemy. Another could be that he travels back in time to change an event in the past that has a major impact on the present in the world of the Gods.
A possible plot for a 'Percy Jackson Secret Power' fanfiction could involve Percy having a secret power of teleportation. But this power is not fully under his control at first. He might accidentally teleport to strange and dangerous places. One time he could end up in the middle of a Titan's lair. As he tries to master this power, he discovers that it has a connection to an ancient prophecy. His friends, like Annabeth, help him figure out how to control it and use it for the greater good.
Well, perhaps in the fanfiction, the Power of Dominion gives Percy the ability to enter the dreams of other characters. He could use this to either gather information or to influence their actions in a positive way. For example, he might enter the dreams of a demigod who is on the verge of turning to the dark side and convince them to stay good. Also, he could use the power to create a safe haven for demigods, a place where the power of Dominion shields them from all outside threats.
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.
One idea could be Percy and his friends getting trapped in a virtual game world that's a twisted version of their own. They have to use their demigod skills to defeat the digital monsters and find a way out.
Maybe Percy is sent on a quest to Atlantis because there are rumors of a traitor among the Atlanteans who plans to side with the Titans. Percy has to navigate the complex political situation in Atlantis and figure out who the traitor is before it's too late. He also has to deal with the Atlantean magic which is different from what he's used to.
One idea could be that Percy discovers a hidden realm within the world of the Wanderer. This realm has its own set of rules and challenges that he has to navigate. Maybe he meets new demigods there who have been lost for centuries.
One idea could be Percy having to make a deal with a minor underworld god to save Annabeth from a new and strange curse that's slowly draining her life force. He has to complete a series of tasks in the underworld that test his loyalty and courage more than ever before.
Well, perhaps the shield was once owned by a powerful demigod from the past. In the fanfiction, Percy starts having strange dreams about this demigod and the shield's true origin. As he investigates, he realizes that the shield holds the key to defeating a new, emerging evil that threatens not just Camp Half - Blood, but the entire world of gods and mortals. So, he has to figure out how to unlock the shield's full potential.
Here's an idea. Emma could be a mortal who accidentally stumbles upon the world of demigods. Percy is tasked with protecting her while also trying to keep the secret of the demigod world from getting out. As time passes, Emma shows an uncanny ability to understand the demigod world, almost as if she has some latent demigod blood in her. They develop a bond as they face various monsters and challenges sent by the gods who are not happy with a mortal being so involved. Their relationship could be a mix of friendship and something more as they go through these experiences together.