Maybe there's a situation where the worlds of Greek mythology and the alien species from Ben 10 (where Gwen is from) start to merge. Percy and Gwen have to figure out how to stop it before chaos ensues. They might face challenges like dealing with gods who are intrigued by the alien technology and aliens who want to harness the power of the Greek gods.
Well, you could write a fanfiction about Gwen's past. For example, how she felt when she first discovered her powers. Was she scared? Excited? You could also write about her relationships within the family, like her interactions with her parents or other relatives. And don't forget about her friendships. Maybe she has a falling - out with one of her friends and then has to work hard to fix it in the fanfiction.
Maybe a plot where Ash's Pikachu gets lost and Gwen, using her magic skills, helps Ash track it down. Along the way, they encounter some new and strange Pokemon species that are attracted to Gwen's magic. They then have to figure out how to get Pikachu back without causing any harm to these new Pokemon.
Another way to develop them is by having them face a moral dilemma together. Say they have to decide whether to save a group of innocent people at the cost of angering a powerful god or alien leader. Through their discussions and the choices they make, their characters can be deepened. Percy might draw on his experiences with the Greek gods' sense of justice, while Gwen could bring in her own values from her world, and they would have to find a common ground.
A possible plot is that there is a threat in both worlds at the same time. Gwen and Harry have to team up, with Gwen using her alien powers and Harry his magic. They travel between the two worlds, trying to find the source of the threat. Gwen's Ghostfreak form could be used to spy on the enemies in the Harry Potter world without being detected easily.
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 a crossover where Naruto and Gwen end up in the same dimension. They might initially be at odds, with Naruto being all about ninjutsu and Gwen having her alien - based powers. But as they face a common enemy, like an evil alien - ninja hybrid, they learn to work together. Their different fighting styles and outlooks on life would make for an interesting dynamic.
Perhaps a fanfiction where Percy and Thalia are transported to an alternate dimension. In this new world, they have to figure out how to get back while also dealing with different versions of their friends and enemies. Thalia might have to face an alternate version of herself who has turned evil, and Percy could be the one to help her see the light again. It would be full of action, drama, and self - discovery.
A plot could involve Hera taking Percy on a journey through the past to show him the origins of the gods' power and how it relates to the current problems in the demigod world. This would give Percy a new perspective and knowledge that he can use to save the day. Or perhaps Percy accidentally stumbles upon a secret of Hera's that she's been hiding for centuries, and she has to enlist his help to keep it safe, all while other gods are trying to uncover it too.
Another plot idea could be that Percy discovers a hidden underwater civilization that is not related to Poseidon. This civilization has its own set of rules and powers. Percy has to navigate through their complex society and politics while also trying to convince them to help the demigods in a coming war against an ancient evil that is rising again. The underwater civilization could have unique magic and technology that the demigods have never seen before.
One idea could be that Percy has to face a new threat from Medusa's long - lost sister who has even more powerful petrifying abilities. He has to use his skills and some newfound knowledge about the Gorgon lineage to defeat her.