Well, if Ash starts with an Ekans, his team dynamic would be very different. Ekans is a snake - like Pokemon, and it might not get along well with some of the more energetic or flighty Pokemon at first. For instance, Pikachu might be a bit wary of it since they are so different in nature.
One possibility is that Ash's Ekans could be a special one with unique abilities. Maybe it has a hidden power that helps Ash in battles against stronger opponents. For example, it could have a super-effective type of attack that is not common among Ekans, like a special poison - type move that can paralyze opponents for a long time.
He might become more cautious. Usually, Ash is quite bold and rushes into things. But if he's in a Team Rocket - centered fanfiction, he'd likely be more careful as he's constantly surrounded by danger. For example, he'd think twice before challenging a new Pokemon or entering an unknown area.
It would change his journey a great deal. Firstly, having a shiny Charmander would make him stand out more. Other trainers would be more likely to challenge him, either to prove themselves against someone with a rare Pokémon or to try and get his Charmander. Also, trainers with more experience might offer to help Ash train his shiny Charmander, which could lead to new training techniques. In Pokémon contests, his shiny Charmander would be a show - stopper, giving him an edge in those aspects of his journey as well.
Growlithe could be a very loyal addition to Ash's team. It might have a more protective nature compared to some of Ash's other Pokemon. This could change the way Ash approaches battles. Instead of always going on the offensive, he might rely on Growlithe to defend the team first. Also, the other Pokemon on the team might look up to Growlithe's courage, which could inspire them to be braver in battles.
He could have a Froakie. It's a common and cute starter in Kalos. Maybe also a Skiddo for traveling across the Kalos landscapes. And a Bunnelby would be a nice addition for its digging abilities.
Ash would likely become more open - minded. Meeting a shiny ghost pokemon at the start would expose him to a type of pokemon he might not have expected so early. This could make him less fearful of the unknown in general. For example, he would be more willing to explore spooky places or deal with pokemon that have a reputation for being difficult to handle.
Ralts could make Ash more empathetic. Since Ralts is a psychic - type Pokémon that can sense emotions, Ash might become better at understanding the feelings of other Pokémon and people. This could help him in his journey to be a better Pokémon trainer.
Ash would likely be more confident from the start. Since he has a powerful Charmander, he might be more eager to take on stronger opponents. He would also have to learn how to handle a Pokemon with such great power, which could make him more responsible.
Well, if Ash starts with Mankey, his whole journey would be altered. Mankey's evolution, Primeape, is much larger and more intimidating than Pikachu. So Ash would probably have to deal with different types of opponents being more cautious around him. Also, training methods for Mankey would be different. Instead of focusing on Pikachu's electric attacks, he'd be working on Mankey's fighting moves like Low Kick and Karate Chop. This would also change the types of Pokemon he might catch later to complement Mankey's abilities.
It would likely bring a new level of harmony (literally) to Ash's team. Since Meloetta has musical powers, it could use its melodies to boost the morale of the other Pokémon on the team. For example, before a big battle, Meloetta could sing a song that makes Pikachu feel more energetic and ready to fight.