Eevee could be the main character. Eevee has so many different evolution possibilities. If it's been around a lot of other evolved Eeveelutions and feels left out or inferior, it could throw a hissy fit. It might run away from its trainer and get into all sorts of adventures while in its angry state, and the trainer has to search for it and figure out how to calm it down.
Meowth from Team Rocket could also be a main character in such a fanfiction. He's always trying to prove himself to his bosses, Jessie and James. If things don't go his way during a mission to steal a powerful Pokemon, he could throw a hissy fit. He might start complaining loudly and making a scene, which could attract unwanted attention from other Pokemon or trainers.
Perhaps a trainer forces a Pokemon to do something it really doesn't want to do, like making a water - type Pokemon go into a really hot area. So the Pokemon throws a hissy fit, causing chaos in the area. It could start using its attacks randomly, and the trainer has to figure out how to calm it down.
It could be a scientist who is involved in the study of Pokemon genetics. While trying to understand the mechanics of evolution better, they accidentally stumble upon the new evolution and then become involved in the events that unfold as a result, both in the scientific and the trainer communities.
It could also be an experienced pokemon trainer who has seen a lot in their travels. After encountering a mirage pokemon by accident and realizing how different and powerful they are, they become determined to become the mastermind in the world of mirage pokemon. They use their existing pokemon training skills and knowledge to study these special pokemon, making them a formidable force in the fanfiction.
The main character could be a talented pokemon stylist. They have a passion for making pokemon look their best with unique haircuts. They travel around different regions to learn new haircut techniques and to show off their skills on various pokemon.
It could be Ash. He's the classic Pokemon protagonist and has been through so many trials already. His will has been tested countless times, like when he faced the Elite Four or when he had to rescue his Pokemon from Team Rocket. In this fanfiction, he could face a new kind of test that really pushes his will to the limit.
A Pikachu could be a great main character. Pikachu is popular and has a lot of potential for character growth. It could start as a timid Pokemon but become a strong leader during the uprising.
Squirtle. Squirtle is cute and friendly. If it adopted a human child, it could show the child the underwater world of Pokemon. The Squirtle could teach the child how to swim, and they could explore underwater caves together. It would be a really fun and light - hearted story with Squirtle as the main Pokemon.
A young girl from a small village in Galar who dreams of becoming a great Pokemon trainer. She could be inspired by the local Gym leader and sets out on her journey with her first Pokemon, a Scorbunny.
Maybe it's a Pokemon itself, like Eevee. Eevee is very friendly and curious. It could be the one to start the whole dancecapades at the party by accidentally finding a magical dance item. Then all the other Pokemon follow Eevee's lead and start dancing, and the story is about Eevee's adventure during this wild party.
It could be a young and inexperienced trainer who is just starting their journey in this new world. They have big dreams but a lot to learn.