A Vulpix might be a good choice for her. It's a Fire - type Pokemon, which shows her having some strength. Vulpix is also very elegant and could symbolize her grace. And in the fanfiction, she could have a special bond with her Vulpix, like it was a gift from a family member when she was young.
One interesting storyline could be that Ash's sister discovers she has a special power related to Pokemon that she has to hide from the world. For example, she can communicate with Legendary Pokemon, and this ability attracts the wrong kind of attention from villains who want to use her for their own gain.
Yes, there are many. Some fanfictions might have him paired with Misty, creating really cute and adventurous storylines where their relationship develops over their Pokemon journeys.
Well, in a fanfiction where Ash has a fever, it could lead to a lot of different plot developments. Maybe Team Rocket would take advantage of his weakened state and try to steal his Pokémon. But of course, his friends, like Misty and Brock, would be there to protect him. They might search for a special herb or medicine in the Pokémon world to cure his fever.
Sure, there are. Often, these fanfictions will give the brother a unique backstory. Maybe he was separated from Ash at a young age and has been on his own Pokemon journey in a different region. Their reunion in the fanfiction could be filled with emotion, and they could then team up to take on powerful Pokemon trainers or face some great danger together.
One possibility could be a magical encounter. Maybe he stumbles upon a rare and powerful Pokemon or a mystical artifact that bestows him with wings. It could also be a result of some genetic experimentation by a mad scientist in the Pokemon world who targets Ash for an experiment gone wrong (or right, in this case of getting wings).
Another theme is enhanced relationships with pokemon. Since he has these instincts, his bond with his pokemon becomes even stronger, which is often a big part of the story. For example, he might be able to communicate with them in a more profound way. And this can lead to more successful battles and adventures.
Another theme is the 'reunion' theme. In fanfictions where Ash is with Serena, after being apart for a while, they meet again and reignite their connection. It often involves a lot of reminiscing and newfound feelings emerging stronger than before.
The fanfiction could focus on Ash's daughter's relationship with her father. Ash could teach her all he knows about Pokemon battles and care. They might go on adventures together, and she could learn valuable lessons from him. For example, how to befriend wild Pokemon or how to strategize in battles. It could also show how Ash has changed as a father compared to his days as a young trainer.
Well, it could be about Ash having a secret power that he's been hiding all along. Maybe it's a special connection to a legendary Pokemon that no one knows about.