Team Rocket could be the main antagonists. They might hear about Ash's encounter with Mew and try to capture Mew for their own evil plans. They could set up all sorts of traps and schemes to try and separate Ash from Mew and steal Mew away.
One idea could be that Ash discovers a young and injured Mew. He takes it upon himself to nurse Mew back to health. During this time, they form a deep bond and Mew starts to teach Ash some unique Pokémon moves that are not commonly known. This new knowledge gives Ash an edge in his battles.
You can try looking on fanfiction websites like FanFiction.net or Archive of Our Own (AO3). They have a large collection of Pokémon fanfictions, and there's a good chance you'll find some 'ash mew starter fanfiction' there.
Well, in some 'ash mew starter fanfiction', the plot could revolve around Mew choosing Ash as its trainer. This is quite interesting because Mew is such a powerful and rare Pokémon. The story might then show how Ash and Mew form a special bond, different from the bonds he has with his other Pokémon. They could face challenges from other trainers who are jealous of Ash having Mew.
Maybe in the fanfic, a mad scientist creates a device that accidentally swaps Ash's essence with Mew's. As Mew - in Ash's body - Ash has to figure out how to reverse the process while also dealing with Team Rocket, who notice something very strange about 'Ash'. Meanwhile, the real Ash, in Mew's form, tries to use Mew's abilities to stop the scientist from causing more chaos.
The plot could start with Ash getting Riolu as his starter Pokémon instead of Pikachu. Riolu might be a bit of a troublemaker at first, not fully understanding Ash's commands. But as they journey together, they start to understand each other better.
I'm not sure specifically as 'ash goomy starter fanfic' could be about a lot of things. It might be a fan - created story where 'Ash' is a character and 'Goomy' is some sort of creature or entity, perhaps a starter in a game or a unique being in a fictional world. But without more context, it's hard to say exactly.
Well, a fanfic with Ash and Riolu as a starter can be really interesting. Riolu has great potential and with Ash as its trainer, there could be a lot of exciting adventures. It might start with Ash finding Riolu in an unexpected place, like a forest or near a stream. Then they'd have to build their bond, which is always a great part of Pokémon stories.
One possible fanfic could be about Ash starting his journey with Snivy. Maybe it focuses on their initial struggles to understand each other. Ash, being used to more energetic starters, might find Snivy's calm and sometimes aloof nature a bit difficult to deal with at first. But as they face various Pokemon battles together, they gradually build a strong bond.
First, do your research on both Ash and Mew. Know their personalities, abilities, and storylines in the Pokémon universe. For example, Ash is a determined and kind - hearted trainer, while Mew is a rare and powerful psychic Pokémon. Then, come up with a unique plot. It could be something like a magical accident that causes the transformation. Next, focus on how Ash would react in Mew's body. He would likely be both excited and a bit scared at first. Use vivid descriptions to bring the story to life, like describing how it feels for Ash to fly as Mew or use its psychic powers. Also, don't forget to include other characters' reactions, such as Pikachu's confusion or Misty's shock.