A really interesting plot could be that the champion of the sun discovers a hidden part of their power. Maybe it's a power that was sealed away long ago. As they start to explore this new power, they also attract the attention of powerful entities who either want to help or stop them, depending on their own motives. This could lead to a journey of self - discovery and intense battles.
One plot idea could be that a powerful Pokemon is corrupted by an Infinity Stone - like object. The Avengers, who are familiar with the power of the Stones, team up with the Pokemon Champions to purify the Pokemon. The Champions know how to communicate with Pokemon on a deeper level, which helps in the process.
One idea could be a story where the protagonist is a trainer who discovers a rare and powerful Pokemon that has been lost for centuries. This Pokemon could have unique abilities that are different from what is known in the regular Pokemon world. Another idea is to focus on a team of trainers who are on a quest to save the Alola region from an unknown threat, perhaps an evil organization that wants to use the power of the Legendary Pokemon for their own gain.
One idea could be Percy having to master Hecate - related magic to defeat a new type of monster that is immune to physical attacks. Another could be Percy being chosen by Hecate to retrieve a powerful magical artifact from the underworld. And perhaps Percy must use Hecate's magic to save his friends who have been trapped in a magical maze created by an old enemy.
One idea could be Ash facing off against old rivals who are jealous of his new Kalos champion status. They challenge him in unique battles that test his skills in new ways.
One idea could be Percy facing a moral dilemma where the will of the Primordials conflicts with his own sense of justice. Another could be him forming an unexpected alliance with a primordial being to save the world from a greater threat.
One idea could be a trainer from a small, unknown town who enters the Pokemon Champion League with an underdog team of Pokemon. Another could be a story about a former champion who comes back to reclaim their title after a long hiatus. Maybe a story where a trainer has to face unexpected challenges during the league, like a new type of Pokemon or a rule change.
One idea could be Ash facing a new, extremely powerful and mysterious Pokémon in the Kalos region that no one has ever seen before. This Pokémon could be the key to a hidden power within the Kalos champion title. Another plot could involve Ash having to team up with a former rival from his past travels to overcome a threat to the Kalos Pokémon ecosystem. Maybe there's a dark organization trying to disrupt the balance, and Ash's unique bond with his Pokémon is what's needed to stop them.
It could involve Percy having to face challenges related to the sun and moon deities. Maybe he has to retrieve a powerful artifact that controls the sun and moon's powers.
One idea could be exploring the backstories of the characters more deeply. Maybe a story about their childhood and how it shaped their relationship as brothers.
Maybe there is a new character, a brilliant scientist, who has developed a dangerous weapon. Shi - Jin and Mo - Yeon have to team up with their friends to stop this weapon from being used, while also dealing with the scientist's personal vendetta against the military. This would add a lot of action and suspense to the fanfiction.