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.
Team Rocket could be the main antagonists. They might see Ash's transformation into Mew as a way to capture a powerful Pokémon - like being. They could devise all sorts of plans to catch him, thinking that they can use Mew - Ash's powers for their own evil purposes.
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.
Perhaps a dark - themed Pokémon organization that is researching the unique powers of Mudkip. They see Ash's Mudkip as a key to their experiments. So they send out their agents to capture it. These agents are skilled trainers with powerful Pokémon at their disposal, and they constantly pose a threat to Ash and his Mudkip throughout the fanfiction.
A powerful and greedy Pokémon collector could be the main antagonist. He wants to capture Ash, now in his Mew form, to add to his collection. He has all sorts of high - tech devices and trained Pokémon to try and catch Ash.
Maybe a powerful and envious Mewtwo could be the antagonist. Since Ash has now become a Mew, Mewtwo might feel threatened or think that Ash doesn't deserve such power. So, Mewtwo could try to fight Ash to prove his own superiority.
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.
It could be a trainer who has a starter that has a type advantage over Ash's new starter. For example, if Ash has a Squirtle and the antagonist has an electric - type starter like Pikachu, they would constantly be in conflict. The antagonist might be jealous of Ash's skills or the potential of his Squirtle and try to undermine him at every turn.
Another possibility is that an old rival of Ash's, who has always been jealous of his success, becomes the main antagonist in this fanfiction. When Ash chooses a different starter, this rival sees it as an opportunity to finally defeat him, thinking that Ash's new and untested starter will be his downfall.
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.