A cool plot could be that the ancient prisoner is part of an ancient pact that the gods made and forgot about. Percy stumbles upon the mystery of the prisoner and realizes that if he doesn't free the prisoner in time, all of the demigods will be in danger. As he tries to free the prisoner, he has to face various challenges set by the gods who don't want the pact to be broken, and also deal with the fact that the prisoner may not be as trustworthy as he seems at first.
Perhaps the ancient prisoner is actually a wronged hero from the past. Percy discovers this and has to fight against the forces that want to keep the prisoner locked up, all while dealing with the internal politics of the demigod world. The prisoner could hold the key to a new kind of magic or weapon that the demigods need to defeat an upcoming evil. It could also be that the prisoner has a connection to Percy's family history, and by freeing him, Percy learns more about his own heritage and powers.
It could be an old Titan who was locked away by the Olympians. Maybe one who was not as evil - minded as the others and was wrongly imprisoned.
One idea could be Percy having to face a new and powerful sea monster that is awakened by an ancient curse. He'd have to use his powers in unique ways, maybe even discovering new abilities as the son of Poseidon. Another could be Percy traveling to a long - lost underwater civilization that has a connection to his heritage as an ancient son of Poseidon.
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.
Well, a cool plot might be Percy getting lost in time and ending up in Ancient Greece during the Trojan War. He could then have to figure out how to get back to his own time while also getting involved in the events of the war. Maybe he tries to change the outcome of the war, but realizes that doing so could have disastrous consequences for the future. Or perhaps he discovers that there's a secret reason why he was sent back in time, like to retrieve an important artifact that was lost during the war.
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.