One idea could be that Bianca doesn't die. Instead, she discovers a hidden power within herself that can control time briefly. This power comes in handy when she and Percy are in a dangerous situation against a powerful monster. They are trapped in a maze - like lair, and Bianca uses her time - controlling power to freeze the monster, allowing them to escape.
Another plot could involve Bianca and Percy going on a secret quest together. They find an ancient prophecy that only the two of them can fulfill. Along the way, they have to face their inner demons. For example, Percy has to deal with his fear of losing Bianca like he lost so many others, and Bianca has to come to terms with her past and the choices she made in the past regarding her brother and the Hunters.
One idea could be that Percy and Bianca team up to go on a secret mission in the Underworld. They have to face various monsters and challenges while also dealing with their own personal growth and the growing bond between them.
One idea could be exploring how Percy and Bianca deal with the unexpected pregnancy in a world full of mythological challenges. Maybe they have to seek help from the gods or other demigods to ensure the safety of the unborn child.
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.
One interesting plot idea could be a crossover where Percy Jackson's demigod powers interact with the magic in The Originals. For example, his ability to control water could be used in a battle against the vampires in New Orleans. Maybe he helps the Mikaelson family during a flood caused by a powerful witch, using his water - based powers to save them and in turn they help him with some demigod - related problem he has, like finding a lost artifact.