For Ash, Pikachu is always a great choice as it's his most loyal and powerful partner. Alongside that, a strong flying type like Charizard could be useful. For Brock, his Onix would be a staple. And perhaps a water - type like Vaporeon could be good for him to balance his team. Since they are teaming up, having a psychic - type like Alakazam could also be beneficial as it can help in battles and also with communication in tricky situations.
One possible strongest team could be Pikachu, Charizard, Greninja, Lucario, Sceptile, and Infernape. Pikachu has been with Ash since the start and has a strong bond. Charizard is extremely powerful. Greninja has its unique form and great combat skills. Lucario is a very capable fighter. Sceptile was a strong addition to Ash's team in Hoenn. And Infernape showed great power in Sinnoh.
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.
A good team could be Pikachu for its popularity and electric type moves which are useful in many situations. Charizard as it's a powerful fire type. And Bulbasaur which can cover different types with its grass and poison attacks. These three are classic and can handle a lot in the story.
Definitely Ash. His journey and how it's affected by the transformation can be the heart of the story. Team Rocket has so much potential. They could be the cause of the transformation, or they could be accidental victims too. Another character worth focusing on is Tracey. He's not as well - known as some of the others, but that could be used to an advantage. He could be the one who discovers a hidden aspect of the transformation. And don't forget about the Pokémon in the Pokémon Center. They could be witnesses to strange happenings related to Ash's transformation and could play a small but important role in the overall fanfiction.
If Team Rocket turns Ash into a Pokemon, it could lead to a really interesting story. Ash would have to experience the world from a Pokemon's perspective. He'd understand their struggles, like being caught and trained. This could also be a way for him to better understand his own Pokemon and their bond with him. And it would be a wild adventure as he tries to find a way to turn back to his human form.
In a fanfiction where Ash joins Team Rocket, it could lead to some really interesting storylines. Ash has a lot of knowledge about Pokémon training that Team Rocket lacks. He could potentially teach them new techniques, but at the same time, his moral compass would be constantly at odds with their devious ways. Maybe he joins them because he's somehow tricked or he has his own secret plan to take them down from the inside. It would be a great opportunity to explore different aspects of his character and how he adapts to this new situation.
One possibility is that Ash joins Team Galactic under some sort of misunderstanding. Maybe he thinks they are a group with good intentions at first. Once he realizes their true nature, he has to figure out a way to escape or stop them. This could create a lot of tension and action - packed scenes in the fanfiction.
A good team could be Sprigatito, Fuecoco, Pawmi, Rookidee, Nacli, and Hoppip. Sprigatito has some useful grass - type moves early on. Fuecoco is a solid fire - type. Pawmi is an electric - type with decent power. Rookidee is a fast flying - type which can be very helpful in battles. Nacli is a rock - type that can take hits well. Hoppip is a grass - and flying - type that can be useful in different situations.
Well, in my opinion, a solid team for the Pokemon Shield story would be the following. Firstly, have a Zamazenta. It's a powerful Legendary Pokemon with great stats. Then, add a Centiskorch. It's a Fire - type with high attack power. Next, include a Grimmsnarl. It has good special attack and can be quite a tank. Also, consider a Duraludon. It's a Steel - type which is great for defense against many types. Finally, have a Mr. Rime for its ice - type moves and its unique look. This combination gives you a good mix of offense, defense, and different types to handle various situations in the story.