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 fanfiction might focus on Ash's personal growth during the training. Professor Oak could be imparting not just Pokemon - related knowledge but also life lessons. For instance, how to be more patient, how to understand the needs of different Pokemon better. This would make Ash a more well - rounded and empathetic trainer. And with these new skills, he could start to form deeper bonds with his Pokemon and have more successful battles.
In this fanfiction, it could be that the training by Professor Oak is a way to prepare Ash for a huge, upcoming threat in the Pokemon world. Oak might have discovered some ancient prophecy or a new, powerful and evil force emerging. So, he's training Ash intensively to be the hero who can face this danger. Ash would then have to learn new moves, new ways to communicate with his Pokemon and also how to team up with other trainers more effectively.