One idea could be that Ash and Serena travel back to the beginning of Ash's Pokémon journey. They could use their knowledge of future events to help Ash catch even more powerful Pokémon right from the start. For example, they might know where to find a rare Pikachu variant and help Ash catch it, changing the whole dynamic of his early battles.
Another plot could be that they travel to a time in the future where Pokémon have evolved in unexpected ways. They then have to adapt quickly to this new world and find a way to get back to their own time. Maybe they meet future versions of themselves who are very different due to the changed circumstances in that future world.
One idea could be that Ash and Serena travel back to the very beginning of Ash's journey. They could use their knowledge of future events to help Ash catch Pokémon more effectively, like getting a Pikachu right away without the initial struggle. Another plot could involve them traveling to a time when a powerful and rare Pokémon was last seen, and they have to work together to find it and protect it from evil organizations.
Maybe they get lost in a mysterious dimension where the rules of Pokémon battling and catching are different. They have to adapt quickly and find a way back to their own world while also helping the Pokémon in this strange dimension. It could be a place where Pokémon can talk, and Ash and Serena have to work with them to find an exit.
A possible plot is that Ash is completely oblivious to Serena's yandere nature at first. Serena starts leaving little gifts for Ash that are laced with a hidden meaning only she understands, like a charm that she believes will keep him close to her. As the story progresses, Ash begins to notice Serena's strange behavior when she gets extremely jealous over small things, like when he just has a friendly chat with a female Pokemon. This leads to a conflict within their relationship as Ash tries to figure out what's going on with Serena.
One idea could be that they embark on a new adventure in a region neither has visited before. They face unique Pokémon and challenges together, which strengthens their bond. For example, they might come across a rare Pokémon that is said to only appear to those with a strong connection. As they work to catch it, they realize their feelings for each other have deepened.
Perhaps the child gets lost in a mysterious forest while trying to follow in their parents' footsteps as a Pokémon trainer. Along the way, they meet various Pokémon that help them find their way back, teaching the child about friendship and the bond between humans and Pokémon. The child could also learn some new skills from these Pokémon encounters.
One idea could be Ash and Serena having a child who shows an extraordinary talent for Pokémon battles at a very young age. The story could focus on how they train and guide their child, just like how Ash was trained by his mentors. Another plot could be them facing a new and powerful villain who threatens their family, and they have to work together with their Pokémon to protect their home and each other.
One interesting plot could be Ash traveling back to the time of the first Pokemon ever discovered. He could witness the origin of Pokemon-human relationships and perhaps influence how they evolved over time. Another idea is Ash traveling to a future time where Pokemon have developed new and strange abilities, and he has to adapt quickly to this new environment. Also, he could go back in time to prevent a great Pokemon disaster that was hinted at in the original series.
Well, perhaps in the fanfiction, Serena could design her own wedding dress inspired by her Pokemon performances. Ash, on the other hand, might get some advice from his old friends like Brock on how to be the perfect groom. And they could have a wedding ceremony where all their Pokemon use their special moves to create a beautiful display in the sky.
One idea could be that they are dancing at a Pokémon festival. As they dance, they remember all their adventures together, and it makes their dance more passionate. The Pokémon around them also start to move in sync, creating a magical atmosphere.
In a more elaborate plot, Ash crossdresses as Serena to infiltrate an all - female Pokemon trainer's secret society. Serena coaches him on how to act like her. As he delves deeper into this society, he uncovers a plot that threatens the entire Pokemon world. He then uses his skills and the element of surprise (since no one expects 'Serena' to be so battle - savvy) to save the day.