It could be a future version of Team Rocket. They might have become more powerful and devious in the future and travel back in time to stop Ash from achieving something important in the past that would prevent their future rise to power. Another option could be a time - traveling Pokemon hunter who wants to capture rare and powerful Pokemon from different time periods, and Ash stands in his way.
During time travel, their relationship could take a new turn. They might discover new sides of each other. If they travel to a time where Pokémon battles were different, Serena could see Ash's adaptability and grow more attracted to him. On the other hand, Ash could see Serena's quick thinking in handling the unfamiliar situations in the past or future, which would make him appreciate her more. It could be a journey of mutual discovery and growing affection.
Their relationship could become stronger as they rely on each other during the challenges of time travel. They might face dangerous situations where they have to protect one another, which would deepen their bond.
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.
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.
One idea could be that Ash and May travel back to the time when the first Pokemon were discovered. They could witness the first encounters between humans and Pokemon and learn about the origin of Pokemon battles. Another plot could involve them traveling to a future where Pokemon have evolved in unexpected ways, and they have to adapt to these new forms of Pokemon. Maybe they also meet their future selves and gain insights into their own destinies.
Pikachu is a very likely candidate. It's Ash's most iconic Pokemon and has a deep bond with him. Whenever Ash is in trouble, Pikachu often steps up to protect him.
Ash could be a Pokémon trainer with a different starting point in this fanfiction. Born in 1987, by 1997 he might have already made a name for himself in local Pokémon battles. He could have a team of Pokémon that he rescued from difficult situations, like a Charmander that was abandoned by its trainer. His story could revolve around giving these Pokémon a second chance and proving that Pokémon don't need to be 'perfect' to be strong.
I'm not sure specifically as there could be many different versions of this fanfiction. But generally, it might involve Ash and May going on adventures through time, perhaps encountering different Pokemon species from various eras, facing challenges related to time - like fixing time - rifts or something.
The impact of Ash's loss in the fanfiction could extend beyond just himself. His Pokémon might also feel demoralized. However, this could also be an opportunity for them to grow stronger together. Ash could start training more intensively, seek new training methods, or even try to catch new Pokémon to strengthen his team.