One possible reason could be that Ash senses a dangerous situation where Pikachu's full power might attract unwanted attention or put them in more danger. For example, if there are powerful and unseen enemies nearby, Ash might hold Pikachu back to keep a low profile.
If Ash is in a position of power in a fandom, they could hold back fanfic that they perceive as low - quality or not in line with the 'spirit' of the original. They might want to maintain a certain standard for the fan - created works. Maybe they think some fanfics are too self - indulgent or don't contribute positively to the overall fandom experience.
It could mean that Ash is somehow preventing or restraining the creation or spread of fanfiction. Maybe Ash has some kind of power or influence in the context where the fanfic exists, like in a particular fandom community, and is using it to hold back the development of fan - created stories.
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.
One possible plot could be that Ash hears of a powerful Pokémon tournament where the winner gets a special wish. He decides to enter with his current team, battles through tough opponents, and in the final, he faces a trainer with a Charizard. After winning, he uses his wish to get his old Charizard back.
You can try looking on fanfiction platforms like FanFiction.net or Archive of Our Own. Just search for 'Ash gets Charizard back' in their search bars and you should find relevant stories.
One popular plot could be Ash going back in time to the very beginning of his Pokémon journey. He might use his future knowledge to catch Pokémon more effectively, like getting a Pikachu right away without the initial reluctance. Another plot could be him traveling back to save a Pokémon from a past disaster that he couldn't prevent in his original timeline. For example, he could go back to stop the destruction of a Pokémon habitat that led to the endangerment of a certain species. A third popular plot might be Ash traveling back to meet his younger self and teach him important lessons about Pokémon training and friendship that he learned over the years.