Well, in the 'ash goes back to kanto fanfic', it could be that Ash goes back to compete in some special Kanto - only Pokémon battles. He might meet some of his old rivals and have epic rematches. He could also discover new secrets about the Pokémon in Kanto that he didn't know before, like hidden abilities or unique evolutionary paths. And of course, he would probably share his experiences from other regions with the people in Kanto.
In the 'ash goes back to kanto fanfic', one possibility is that Ash returns to help deal with a new threat in Kanto. This threat could be endangering the Pokémon or the people there. He would bring his skills and knowledge from his travels to combat it. Also, he could introduce some of the Pokémon species from other regions to the Kanto region, which would be really exciting for the local Pokémon trainers. And perhaps he'll finally get to fully explore some parts of Kanto that he didn't have time for during his first journey.
He might meet Gary again. They could have a friendly chat or even a Pokémon battle.
One possibility is that Ash would be a bit of a star at the Pokemon school. Since he has so much experience traveling and battling. He could inspire other students to be more adventurous with their Pokemon. Also, there could be some unique Pokemon species that are only studied at the school which Ash gets to interact with.
Well, in fanfiction, they might have an epic Pokémon battle. Ash could show off his unique battle strategies that he's developed over his journey, and Cynthia would bring her powerful Pokémon and tough battling skills. Maybe Ash's Pikachu would face off against Cynthia's Garchomp, creating a really exciting and intense showdown.
In the 'Percy goes back in time fanfic', he may also meet his younger self. This could lead to some really interesting and confusing situations. He would have to be careful not to disrupt the timeline too much while still achieving whatever his goal is for going back in time.
The fanfic could focus on Ash and Froakie's bond in Kanto. Maybe they get separated in a dense forest in Kanto and have to find their way back to each other. Along the way, Froakie discovers its hidden strengths and Ash learns more about the unique Pokémon in Kanto that he hadn't known before.
She could change the whole dynamic of Elijah's life. For instance, if Elijah was on a path of revenge in his original timeline, Bella might be able to show him a different way. She could introduce him to new ideas and values that were not present in his time. This could lead to a completely different version of Elijah, one who is more open - minded and less consumed by his past grudges.
Ash might encounter new Pokémon in Galar. For example, he could come across Grookey, Scorbunny or Sobble, the starter Pokémon of Galar. He may also meet new trainers and have exciting battles with them.
In an 'Ash goes to school fanfiction', Ash might make new friends at school. He could encounter different Pokémon - related challenges in a school setting, like having Pokémon battles as part of a school competition.
In this fanfic, Ash's win in the Kanto League could be due to his growth as a trainer. He has learned from his previous losses and has trained his Pokémon to be stronger. Maybe he discovers a new ability of one of his Pokémon during the league battles. His win could also lead to new opportunities, like being invited to exclusive Pokémon training events or getting the chance to meet legendary Pokémon trainers. It could also show his rivals realizing that Ash is a force to be reckoned with and starting to respect him more.
Imagine Bella goes back in time and meets Jasper when he was in a very different state of mind compared to when she knew him in her own time. This could create a lot of tension and also a great opportunity for character growth. She might have to prove herself to him. And as they interact, they could start to fall in love in a completely new way, not influenced by the events that brought them together in the original story.