Well, a very frequent theme is self - discovery. In these fanfictions, Ash being reborn gives him the opportunity to understand himself better. He might realize his true potential or find new ways to interact with his Pokemon. Also, the theme of new beginnings is quite common. It's like he's starting his journey all over again, but with the wisdom of his past experiences. This allows for new relationships to form and old ones to be re - evaluated. There can also be a theme of facing old foes in a new light, now that he has a different perspective after being reborn.
One popular Ash Reborn fanfiction could be those that explore Ash's further adventures in the Reborn region, like 'Ash's New Beginnings in Reborn'. It might focus on his battles with unique gym leaders there.
One possible plot could be that Naruto, with his ninja skills and determination, takes on the Pokemon world as Ash. He might use his combat experience to develop unique battle strategies in Pokemon battles. For example, he could incorporate his shadow clone jutsu in some way, like using clones to distract opponents' Pokemon while his own Pokemon attacks from an unexpected angle.
One possible plot could be that the OC, now as Ash, has all the knowledge from their previous life. They might use this to train their Pokémon more effectively. For example, they could quickly figure out the best training regime for Pikachu, making it even stronger than in the original series. They could also have different strategies in Pokémon battles, using their prior knowledge of different Pokémon types and moves to their advantage.
Obviously, Ash himself is a central and popular character. Since the whole premise is being reborn as him, his actions and growth are key. Pikachu is also extremely popular as it's Ash's constant companion. Another popular character could be Gary, as his rivalry with Ash is a significant part of the Pokémon story and can be explored in new ways in this type of fanfiction.
One possible plotline could be the protagonist, reborn as Ash, using their prior knowledge to quickly become a more skilled Pokémon trainer. For example, they might know which Pokémon are strong in certain situations and target those for capture first. Another plotline could involve the protagonist trying to change the fate of some of the Pokémon characters in the original story, like preventing certain Pokémon from getting injured in battles.
One popular 'ash is reborn fanfiction' is 'Ash's New Beginning'. It shows Ash starting his journey all over again with new skills and a more mature mindset. Another great one could be 'Reborn Ash: The Unstoppable Trainer' which focuses on Ash's rapid growth and his ability to overcome tougher challenges this time around.
One challenge could be living in Ash's shadow. Since Ash is so well - known in the Pokémon world, it might be hard for the character to be recognized for their own achievements. Another challenge could be dealing with Ash's impulsive nature. If the character is more cautious, they might constantly have to balance Ash's eagerness to jump into things. Also, they might face challenges from other trainers who assume they are just like Ash or are trying to use them to get to Ash.
Naruto's chakra control could be related to how Ash controls the energy flow in his Pokemon. Maybe he could use his chakra knowledge to enhance his Pokemon's attacks or defenses. Also, his ability to learn jutsus quickly might turn into an ability for Ash to quickly understand and train his Pokemon for new moves.
To start writing Ash Reborn fanfiction, think about the characters you want to include. You can introduce new characters or develop the existing ones further. For example, you could give more backstory to a Reborn gym leader. And don't forget to plan the flow of your story, from the beginning, through the middle, to the end.