Perhaps the Dark Lord creates a powerful spell that makes all the demigods forget their powers. Percy discovers that he is immune to this spell due to his connection with Poseidon. He then has to go on a quest to find the source of the spell and break it. Along the way, he meets old friends who don't recognize him and he has to figure out how to convince them to trust him without their memories.
One idea could be that the Dark Lord somehow manages to corrupt the powers of the gods. Percy has to find a way to stop this corruption before it spreads across the whole world of demigods. He might have to team up with some unexpected allies, like a group of monsters who also don't want to see the world destroyed in this new, chaotic way.
The Dark Lord could be planning to merge the mortal world and the world of the gods in a way that would cause mass destruction. Percy is the only one who realizes the danger as he has had experiences in both worlds. He must find a way to stop this merger. This could involve him having to enter the Dark Lord's lair, which is filled with all kinds of dangerous traps and dark magic, and face the Dark Lord in a final showdown.
One idea could be Percy discovers a new underwater civilization that is in danger from an ancient sea monster. He has to use his powers and wits to save them.
Perhaps there could be a plot where a powerful and dark gamer is trying to take over the world through the gaming platforms, and Percy, along with his demigod friends, has to enter the games to stop this evil plan. They could face challenges like corrupted game levels, glitched enemies that are super - powerful, and a race against time to prevent the dark gamer from achieving their goal.
Perhaps an OC who was turned dark due to a curse. They could start off as a normal demigod but after being cursed by a powerful enemy, they gradually become more malevolent. Their journey could involve trying to break the curse while also being hunted by other good - minded demigods.
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.