A more emotional plot could be where Snivy gets separated from its group and is found by Servine. Servine takes Snivy under its wing. As they travel together, they face misunderstandings from other Pokemon because of their different natures. But through it all, they develop a strong bond that helps them face any difficulties.
For Snivy, you can show its growth from being a bit naive to more confident. For example, at the start, it might be hesitant in battles, but as the story progresses, it becomes bolder. Servine can be developed as a more mysterious character at first, with its past gradually being revealed as it gets closer to Snivy.
Maybe Snivy is a wild Pokémon and Servine is a Pokémon that has been with a trainer for a long time. Servine gets lost during a journey and Snivy helps it find its way back. During this time, they fall in love. This plot can explore the differences between wild and trained Pokémon's worlds.
You can try some popular fanfiction websites like FanFiction.net or Archive of Our Own. They usually have a large collection of Pokémon - related fanfictions, including ones about Snivy and Servine's relationship.
A powerful and greedy trainer could be the main antagonist. This trainer wants to capture Servine and Charmeleon not for the love of Pokémon but to sell them for a high price in the black market. He constantly chases them, setting up traps and using other Pokémon to try and weaken them. This creates a lot of tension in the story as Servine and Charmeleon have to constantly be on the run and find ways to outwit the trainer.
Maybe Servine and Charmeleon are both captured by the same trainer who has a bit of a rough training style. They initially dislike each other because they think the other is getting more attention from the trainer. But one day, when the trainer is too harsh on them during a training session and they run away together. In the wild, they have to learn to survive without the trainer's help. They start sharing food, finding shelter together. As they face the challenges of the wild, they realize that they are stronger together and form an unbreakable bond.
A common theme is friendship. They are from different elemental types, so the fanfic often focuses on how they overcome their differences to become friends. For example, in many stories, oshawott might be more impulsive while snivy is more calculating, and they learn to balance each other out.
The setting is also important. It could be in a bustling Pokémon city, a quiet rural area, or a dangerous wilderness. The setting affects the kind of challenges they face. For instance, in a city, they might have to deal with human - Pokémon relations, while in the wilderness, it could be about surviving against wild Pokémon.
You can try looking on fanfiction.net. It has a large collection of Pokémon fanfics, so there's a good chance you'll find some great 'ash x snivy' ones there.