I'm not sure specifically as there could be many different 'ash's snivy evolves' fanfics. Generally, it might be about the evolution of Snivy in Ash's team. It could involve the journey leading up to the evolution, like the battles and experiences that trigger it.
In some fanfictions where Ash's Gible evolves, it might gain new and powerful moves. For example, it could learn Dragon Claw and become a more formidable opponent in battles against other trainers' Pokémon. It might also develop a closer bond with Ash as it evolves, showing more loyalty and determination to win battles for him.
The main characters would be Ash and Gible without a doubt. Ash as the trainer is responsible for Gible's growth and development. Gible, on the other hand, is the focus of the evolution part. However, depending on the fanfiction, other trainers that Ash competes against could also be considered main characters. They might be envious of Ash's Gible evolving or try to come up with strategies to defeat it once it has evolved.
You can try looking on fanfic websites like FanFiction.net or Archive of Our Own. They have a large collection of Pokémon fanfics and there's a good chance you'll find 'ash's snivy evolves' fanfic there.
If Pikachu evolves in a fanfiction, it might have an impact on Ash's relationships with other characters. Some trainers might see Ash as a more serious threat now that his Pikachu has evolved. Others might be jealous or try to figure out how he achieved such a transformation.
In a fanfiction about Pikachu evolving, it might happen because of a strong emotional bond. Suppose Pikachu is trying to protect Ash from a great danger. In that moment of extreme self - sacrifice and love for Ash, it evolves. This evolution could lead to a whole new dynamic in their relationship. Ash would have to learn how to train a Raichu instead of a Pikachu, and Raichu might be more independent but still deeply loyal to Ash.
Well, if Ash's Pikachu evolved into Raichu in fanfiction, it could change the whole dynamic of their journey. Pikachu's evolution might bring new power levels and moves, making battles easier or more complex. Ash would need to adjust to the new Raichu, and their bond might be affected. Maybe Pikachu's cute factor would be replaced with a more intimidating Raichu presence. It could also lead to new storylines where Ash has to deal with the jealousy of other Pokemon or trainers who are envious of his powerful Raichu.
One possibility is that Ash has to quickly adjust to Raichu's new power levels. Raichu might be a bit more independent compared to Pikachu, which could lead to some initial clashes in their teamwork. But in the end, they could become an even more formidable team, with Raichu's increased strength allowing them to take on tougher opponents in battles.
One common theme is the underdog story. Magikarp is often seen as a weak Pokémon, so many fan fictions focus on it rising above its limitations. Another theme is the relationship between the trainer and Magikarp.
Maybe a shiny Magikarp is found by a group of researchers who believe it has special powers. They study it, but the Magikarp is not happy being in a lab. It befriends a young intern who helps it escape. Together, they journey through the Pokemon world, facing various challenges and making new friends along the way. This plot could explore themes of friendship and freedom.