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.
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.
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.
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.
Perhaps an ancient evil spirit that was sealed away long ago. The hatching of the legendary starter's egg is a sign that the spirit can break free. It wants to possess the starter to regain its full power and take over the Pokemon world. This would make for a very dark and menacing antagonist, and Ash would have to use all his skills and the power of his friendship to defeat it.
In this fanfiction, perhaps Ash starts with a Eevee. Since Eevee can evolve into multiple forms, it would add a whole new dimension to his journey. He could face different challenges as he tries to decide which evolution path to take for Eevee, and it would also attract different kinds of Pokémon opponents.
If Ash had a different starter like Bulbasaur in fanfiction, his team composition would change significantly. He might focus more on grass - type Pokemon to complement Bulbasaur's abilities. His travels through different regions could be affected as well. Maybe he would be more drawn to areas with more plant - life where Bulbasaur could thrive. Also, his interactions with other trainers having fire - type starters would be more interesting due to type advantages and disadvantages.
One idea could be to have Ash start with a different type of Pokémon that is not commonly chosen in the main series, like a Skorupi. The story could revolve around the challenges he faces in training it as it has a somewhat menacing appearance and is a Poison - type, which might make some people in the Pokémon world a bit wary. This would lead to interesting interactions with other trainers and gym leaders.
Well, if it's 'Ash gets a different starter fanfiction', perhaps he gets a Bulbasaur as his starter. This would lead to him being more involved in grass - type Pokémon battles and strategies. He could end up exploring different regions in search of rare grass - type Pokémon to strengthen his team. Also, his personality might develop in a different way, as Bulbasaur has a calmer and more patient nature compared to some other starters, which could influence Ash's own temperament over time.
It could be a new character, a powerful trainer who is jealous of Ash's natural talent and secretly tries to undermine him at every turn.
It could be a new dark wizard that is specifically drawn to Harry because of his different Patronus. This wizard might see Harry's unique Patronus as a threat to his own plans for power or chaos in the magical world.