One reason could be for the sake of adventure. If he's immortal, he can go on countless adventures without the worry of dying. It allows the story to be more continuous and expansive.
Well, it depends on the specific fanfic. There are a ton of different interpretations in the fanfic world. Some fanfic authors might give Ash immortality as a way to give him a unique power or status. It could also be part of a more complex plot where he has to endure through the ages for a particular purpose, like protecting the Pokémon world indefinitely. However, not all fanfics will take this route. There are many that keep his character more in line with the original concept where he is mortal.
One reason could be that some writers want to create shock value. By showing Ash Ketchum being abused, they think they can get more attention from readers. Another reason might be that they are exploring different aspects of human nature in a very extreme way, even if it doesn't fit the original character's concept.
One reason could be to pursue a new and more challenging adventure outside of the usual Pokémon journey. Maybe he feels he has achieved all he can in his current region and wants to explore uncharted territories where Pokémon training is different or more extreme.
One good fanfic could be 'Ash's Homecoming'. It focuses on Ash returning home to Pallet Town and his heartwarming interactions with Delia. It shows their strong bond through simple daily activities like having dinner together and sharing stories of his adventures.
There are many family - friendly Ash Ketchum fanfics. Some might focus on his adventures in different regions, like his journey in the Alola region where he forms new friendships with Pokémon and trainers. These fanfics can be about his growth as a trainer, how he overcomes challenges in battles, and his bond with Pikachu in new and exciting scenarios.
An interesting plot is key. For example, if it involves Ash going on a new journey to a far - off region, or facing a powerful and unknown enemy. It should keep readers on their toes. Also, good character development for Ash and his Pokémon is important. Maybe Pikachu learns a new move in a creative way in the fanfic.
In a fanfic, Ash could be turned into a vampire by an ancient, powerful Pokemon with vampiric abilities. This would completely change his relationship with his other Pokemon. Some might be scared of him, while others would stay by his side. He would then have to learn to control his vampire powers and instincts in order to continue his journey. For example, he'd need to figure out how to be in the sunlight without getting burned as he travels from place to place.
I like 'Ash Ketchum's Hidden Powers Fanfic'. In this one, Ash discovers some hidden powers within himself that he never knew he had. It's exciting because it takes the story in a new direction, different from the regular Pokémon anime plot.
Overcoming challenges together is a theme as well. Just like in the original series where Ash faces various Pokemon - related challenges, in these fanfics, he and his love interest face challenges related to their relationship, such as dealing with other people's opinions or finding time for each other while on their respective Pokemon journeys.
It could be about Ash encountering Giratina in a strange new region. Maybe Giratina is causing some sort of chaos, and Ash has to figure out how to calm it down and understand its motives.