A great story could involve Allister traveling to an ancient, haunted place in search of a rare and powerful ghost - type Pokemon. Along the way, he has to face his own fears and learn more about the history of the Pokemon world. He could encounter various challenges like solving puzzles related to the ghosts of the past trainers who once visited that place.
Perhaps a fanfiction where Allister has to compete in a special, spooky - themed Pokemon tournament. All the battles take place in haunted arenas, and he has to use his unique skills and the bond with his Pokemon in new ways. He might also meet some interesting characters during this tournament, both friendly rivals and mysterious spectators.
Perhaps a story where Allister is having trouble with his confidence in battles, and Bea decides to train him. She shows him new fighting techniques for his Pokemon, and in return, Allister teaches her how to better understand the more mysterious aspects of Pokemon battles. This way, they both grow as trainers and form a strong friendship that helps them face tougher opponents in the future.
His backstory could be explored more. For example, we could learn about how he got interested in ghost - type Pokemon in the first place. Maybe he had an encounter with a kind - hearted ghost - type when he was young that inspired him.
One idea could be a story about a trainer who discovers a new, hidden ability in their Pokemon that changes the way battles are fought in the Galar region. For example, a Pikachu that can use a special electric move that is only possible in certain areas of the wild area.
In some fanfictions, the theme of discovery is present. They might find a new type of Pokemon or a hidden power within their existing Pokemon. There could also be a theme of mystery, for example, if they encounter a strange phenomenon in the Pokemon world that they need to solve. And of course, the theme of loyalty between them and their Pokemon is often explored, showing how their Pokemon support them in different situations.
One interesting Allister Pokemon fanfic could be 'Allister's Ghostly Adventures'. It might explore his unique relationship with Ghost - type Pokemon in more depth, like his encounters with different spirits in the Pokemon world. Another could be 'Allister's Hidden Powers', which delves into any potential special abilities he has that haven't been fully explored in the official games or anime.
A fanfiction could focus on the backstory of a Gym Leader in Pokemon Sword and Shield. Maybe they had a difficult past that led them to become a Gym Leader and how they overcame those challenges. It could also involve their relationship with their Pokemon.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). There are often a lot of Pokemon fanfictions there, and you can specifically search for ones involving Bea and Allister. Another option is FanFiction.net. Just use their search function and type in the relevant keywords.
One idea could be to focus on a new trainer's journey in the Galar region. They could start with a unique Pokemon not often chosen, like a Galarian Zigzagoon, and face various challenges that the game doesn't cover. Another could be a story about the backstory of a Gym Leader, perhaps their rise to power and the secrets they hold. Maybe a fanfic where Pokemon from different regions end up in Galar and how they interact with the local Pokemon and trainers.
In Pokemon Shield for the story, you could build a team around Snorlax. It has high HP and can be a tank. Add to that Alcremie which is a Fairy - type with some nice moves. Mr. Rime, an Ice / Psychic type that can be quite strategic in battles. Dubwool, an evolved form of Wooloo, is stronger and can hold its own. And for a fast attacker, Ninjask which is a Bug - type with extremely high speed.
A well - crafted plot involving Allister's Pokemon battles is also appealing. For instance, if the battles are described in great detail, with the use of different Pokemon moves and strategies, it can be very engaging. It could also include unexpected twists in the battles, like a new move his Pokemon learns at a crucial moment.