Well, it could be about Ash getting some really special and secret training from Professor Oak. Maybe Oak teaches him some unique battle strategies that no one else knows. This could lead Ash to become an even more formidable trainer in the Pokemon world.
The appeal might be the unique dynamic. Delia is Ash's mother and Oak is a respected figure in the Pokemon world. In fanfiction, it could be about exploring a different side of their relationships, perhaps a romantic or a more complex emotional bond that isn't shown in the original series. It gives fans the chance to be creative and rewrite the characters in a new light.
One possible reason could be that in the fanfiction, Ash might be depicted in a way that goes against Professor Oak's character values. For example, if Ash is shown as disrespectful to the Pokémon world's traditions which Professor Oak holds dear.
Another possible main character could be Gary. Since Gary has a connection to both Ash and Professor Oak, he could play a role. Maybe he's jealous of Ash getting special training from Oak and tries to interfere or he could also be inspired by Ash's training and start his own journey of improvement. Also, Misty or Brock could be involved as they are Ash's friends and might visit during the training to see how he's doing.
Ash could be paired with Professor Oak in a fan fic. Ash could return to Pallet Town and have a series of adventures with the professor, like helping him with a new research project on Mega Evolutions. They could also travel together to a new and uncharted region.
A plot could revolve around Professor Oak getting involved in a time - traveling adventure. Maybe he accidentally travels back to the days when Pokémon were first being discovered and has to figure out how to get back to his own time while also making sure not to disrupt the course of Pokémon history too much. He could also meet his younger self during this journey.
The fanfiction might also focus on Ash's personal growth as he works closely with Professor Oak. Maybe Oak teaches Ash more about Pokémon history and lore, which in turn helps Ash become a better trainer. For example, learning about the ancient powers of certain Pokémon could give Ash an edge in battles. Ash could then share this knowledge with his friends and other trainers.
Obviously, the main characters would be Professor Oak and Ash's mom. Professor Oak is known for his vast knowledge of Pokemon, and Ash's mom is important as she is Ash's mother and has supported his Pokemon journey from the start.
One possible plot could be that Professor Oak discovers a rare Pokemon that he believes Ash's mom might be interested in. He then takes it to her to show her, and they end up having a long conversation about Ash's adventures and how proud they both are of him.
Another aspect that makes it appealing is the exploration of different themes. It could be about growing up, as Ash matures under Oak's influence. Or it could be about friendship that defies age and social roles. Some fanfictions also play with the idea of time - travel, where Oak and Ash interact in different time periods, adding a whole new layer of intrigue to their relationship. It's this flexibility and the ability to re - imagine their relationship that draws people to 'professor oak x ash fanfiction'.