Well, in such a fanfiction, Ash might gain vampire powers like super strength and enhanced speed. He could use these new abilities to battle stronger Pokemon or face new challenges in the Pokemon world. Maybe he would have to deal with his new thirst for blood while still trying to be a Pokemon master.
His Pokemon could be his potential allies. They've been with him through thick and thin and would likely still support him even with his new vampire state. As for enemies, there might be those in the Pokemon world who fear or dislike vampires and would see Ash as a threat. Maybe some powerful and evil Pokemon trainers who want to use his new situation against him.
In many fanfictions, Ash achieving the status of a Pokemon Master often involves him having extraordinary battles. He might defeat powerful legendary Pokemon that no one has ever tamed before. His Pikachu could evolve into a more powerful form, which gives him an edge in battles. Also, his bond with all his Pokemon, not just Pikachu, becomes so strong that they can execute complex strategies without any communication barriers.
Well, in such a fanfiction, Ash might experience the life of a Pokemon in a very different way. He could understand their thoughts and feelings more deeply. Maybe he'd have to learn how to use Pokemon moves in his new form, which would be a really strange and exciting adventure for him.
It could be a powerful and evil Pokemon trainer who wants to capture Ash in his new Pokemon form. This trainer might be after Ash because he believes that a human - turned - Pokemon has some special power or value.
One common theme is identity. Ash has to figure out who he is now as a vampire in the Pokemon world. Another is friendship. His friends' reactions and how they support or reject him can be a big part of the story.
His Pikachu would probably be very supportive. It would stay by his side as he studies, maybe even help him in small ways like fetching medical tools. It trusts Ash completely and would be happy to see him pursue something new.
A frequent theme in such fanfiction could be adventure. Even as Ashley, the character would still be involved in Pokemon - related adventures. But now, with a different gender, there could be new challenges and opportunities. For example, interactions with other characters, especially of the opposite gender, might change. Also, the theme of self - discovery is quite common. Ashley would need to figure out who she is in this new form and how she fits into the Pokemon world.
One key element could be Ash's character development. In fanfiction, he might face new and unique challenges that further shape him into a Pokemon master. For example, he could encounter extremely rare Pokemon species that test his skills in ways the original series didn't. Another element is the relationships. Maybe his bond with Pikachu deepens even more, or he forms new alliances with other trainers. The setting also plays a role. It could be set in unexplored regions of the Pokemon world, full of new mysteries and Pokemon to discover.
Obviously, Ash (as a girl) would be the main character. Pikachu would still be by her side, being her loyal and cute partner. Her friends like Brock and Misty would also be important characters. Brock might be even more protective of her now, and Misty could become a sort of role model for her in some ways.