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.
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.
One idea could be that as Ash's brother, you have a special ability that Ash doesn't. For example, you can communicate with a certain type of Pokémon that is very rare. This ability helps Ash in his journey in unexpected ways, like leading him to a hidden Pokémon sanctuary where he can catch some unique Pokémon. Another plot could involve you having a different approach to Pokémon training compared to Ash. While Ash is more about friendship and instinct, you focus on strategy and research, and the two of you combine your methods to become even stronger trainers.
Ash could be a Pokémon trainer with a different starting point in this fanfiction. Born in 1987, by 1997 he might have already made a name for himself in local Pokémon battles. He could have a team of Pokémon that he rescued from difficult situations, like a Charmander that was abandoned by its trainer. His story could revolve around giving these Pokémon a second chance and proving that Pokémon don't need to be 'perfect' to be strong.
In a fanfiction about Ash born in 1987 and his situation in 1997, we could see an alternate story where Ash is not the typical rookie trainer. Since he's a bit older than in the original, he might be more knowledgeable about Pokémon lore. He could be on a quest to uncover some ancient Pokémon secrets in 1997. He might travel to different regions, not just for battles but also to study old Pokémon texts and artifacts, and along the way, form an unusual team of Pokémon that are attracted to his in - depth knowledge.
Pikachu would be a likely candidate. Pikachu is Ash's most loyal Pokémon and has a very strong bond with him. It would sense that Ash is in trouble and would go on a mission to rescue him. Pikachu could use its electric attacks to fight off Gary or any obstacles in the way.
In some fanfictions, Ash can touch Pokémon to show a deeper bond. It might be a way for the author to create more emotional and physical interactions between Ash and his Pokémon pals, different from the normal anime where physical contact like hugs is not as common.