Well, perhaps the fanfiction could start with Ash already having a secret girlfriend. She is a Pokémon researcher. Ash has been keeping their relationship under wraps because he doesn't want to draw too much attention. But then, his Pokémon start to notice something is different about him. They begin to act in strange ways, like Pikachu being overly protective of Ash when his girlfriend is around. Eventually, Ash has to decide whether to make their relationship public or not.
One possible plot could be that Ash and his girlfriend go on a Pokémon journey together. They encounter rare Pokémon and have to work as a team to catch them. Along the way, they face challenges from rival trainers who are jealous of their relationship and try to break them apart. But through their love and trust in each other, they overcome all difficulties.
Maybe it's a new character created specifically for the fanfiction. A girl from a far - off region who Ash meets during one of his journeys. She could be a powerful trainer or have some special abilities that draw Ash to her, and they start a secret relationship.
It could be Misty. They have a long - standing relationship in the Pokemon series. They've traveled together and shared many adventures, so in a fanfiction, it's not a stretch to imagine her as a secret girlfriend.
Another possibility is that it's a Pokémon in human form, which is a common trope in some fanfics. This Pokémon - turned - human could have fallen in love with Ash during their adventures together, and now they have to keep their relationship a secret from the world of Pokémon and humans alike.
The plot could be that Percy's girlfriend is a time - traveler. She comes from the future and warns Percy about a great danger that is going to befall his world. Percy is initially skeptical but soon realizes she is telling the truth. They then embark on a journey to stop this danger. Along the way, their relationship deepens as they rely on each other's strengths. Percy uses his combat skills and the girlfriend uses her knowledge of the future to outwit their enemies.
Well, it could be about Ash having a secret power that he's been hiding all along. Maybe it's a special connection to a legendary Pokemon that no one knows about.
In this fanfiction, perhaps Ash starts with a Eevee. Since Eevee can evolve into multiple forms, it would add a whole new dimension to his journey. He could face different challenges as he tries to decide which evolution path to take for Eevee, and it would also attract different kinds of Pokémon opponents.
One possible plot could be that Ash's shiny Pikachu is targeted by an evil organization because of its rarity. Ash has to protect it while also trying to figure out why the organization wants it so badly.
One possible plot could be that the brother is a more experienced trainer and he helps Ash to train his Pikachu to reach new levels of power. For instance, he could teach Ash some special training techniques that are unique to their family.
Misty could be a great choice. They have a long history together, and there have always been hints of a deeper connection between them. Their constant bickering could be seen as a form of flirting in the context of a fanfiction where they finally realize their feelings and start a relationship.
Well, it could also be about the challenges they face in keeping the relationship under wraps. Such as dealing with jealousy from other people who like Ash or his girlfriend. And there could be a plot where they almost get caught, but they manage to come up with a clever cover - up at the last minute.