One idea could be that the forgotten promise was a pact made between two childhood friends to always protect a special place in their town. As they grow up, one moves away and forgets. The other could be struggling to keep the place safe alone and when they reunite, the memory slowly comes back. This could lead to a series of adventures to save the place from developers or some other threat.
One idea could be exploring the backstory of the captain before the promise was made. Maybe there was a tragic event in their past that led them to make such a solemn vow. Another could be a situation where the captain has to break the promise to save their crew, and then deal with the moral and emotional consequences.
One idea could be that the forgotten daughter discovers she has a special power. Maybe she can communicate with animals and this ability helps her find her true place in the world.
One idea could be exploring the backstory of the forgotten warrior more in - depth. Maybe they were once a great hero in a forgotten era of the Percy Jackson universe and got lost in time somehow. Another could be that the forgotten warrior has a special power that is now needed to defeat a new, emerging threat. They could be a key to unlocking an ancient prophecy that no one has understood until now.
One idea could be that the forgotten one has a special power that is the key to saving the world from a new, unknown threat. Another could be that they are actually a long - lost relative of Percy's and their connection reveals hidden secrets about the demigod world. Maybe the forgotten one was sent away on purpose by an enemy long ago and now they're back for revenge.
Maybe in the fanfiction, Percy stumbles upon a forgotten legend about a magical artifact that has the ability to rewrite the fates of demigods. The catch is that it's hidden in a realm that is slowly being consumed by chaos. Percy, along with his friends, must navigate this dangerous realm filled with strange creatures and illusions to find the artifact before it falls into the wrong hands. They'll also have to deal with the fact that the gods are not too keen on this artifact being found as it could disrupt their carefully crafted plans for the demigods.
One idea could be a crossover where Harry Potter characters are transported to the Forgotten Realms. They could encounter different magical creatures there, like beholders or mind flayers. Harry might use his skills to deal with the new threats.
In 'percy jackson forgotten primordial fanfiction', there could be the idea of Percy having to deal with primordial forces that were previously unknown. This could lead to new alliances and challenges. Maybe he has to find a way to communicate with these forgotten entities, which would be a whole new aspect compared to his usual battles with known monsters and gods.
It could be two best friends. They've been through a lot together and the forgotten promise between them creates a rift that they need to overcome. For example, they might have promised to always be honest with each other, but one of them broke it unknowingly, leading to misunderstandings.
Maybe Percy is sent to Russia by the gods as a form of punishment or test. While there, he gets involved in a battle between two ancient Russian magical forces that he has to navigate to survive. Or perhaps he travels to Russia in search of an artifact that can help him in his ongoing fight against the forces of evil back home.
One idea could be exploring the unspoken stories of the ranch hands. Maybe there's a hidden love affair between two of them that has to be kept secret due to the social norms of 1923. Another could be focusing on how a young character dreams of leaving the ranch and going to the big city, but faces many obstacles. And a third idea could be a mystery plot where strange things start happening on the ranch and the characters have to figure out what's going on.