Perhaps in this fanfiction, Ash is betrayed by a mentor figure. This person has been guiding him all along but has a hidden agenda. The rated m aspect could mean there are some intense battle scenes and moral dilemmas. Ash will have to reevaluate his beliefs and skills to overcome this betrayal and the challenges that come with it.
His rival, Gary, could be a likely candidate. Gary has always been competitive with Ash, and in this fanfiction, his jealousy could reach a boiling point, causing him to betray Ash for his own gain, like trying to steal his Pokémon or get credit for his achievements.
One possible plot could be that Ash, being from a different world, is initially welcomed into the RWBY universe. But then, he might be betrayed when his Pokémon powers are seen as a threat. Maybe the RWBY characters fear that his abilities could disrupt the balance of their world. For example, if his Pikachu's electricity causes unexpected power surges in a world that has its own unique energy sources like Dust.
Well, without actually reading the specific fanfiction, it could be about Ash Ketchum, who is in a 'godlike' state, being betrayed by someone. Maybe his friends turn against him due to jealousy of his newfound power. Or perhaps an enemy manages to deceive those close to him, making them think Ash has done something wrong when he hasn't.
A possible plot could be Ash visiting an old, abandoned Pokémon sanctuary. He explores the place quietly, feeling a sense of melancholy. As he walks through the overgrown paths, he discovers an injured and rare Pokémon. He then uses his knowledge and skills in a very calm and methodical way to nurse it back to health. This shows a different side of Ash, one that is more patient and less about the big battles.
One common theme could be Ash being betrayed by his friends in the Pokémon world. For example, they might leave him for a more powerful trainer or because of a misunderstanding. Another theme could be betrayal by the Pokémon League, like being wrongly disqualified from a major tournament. Also, some fanfictions may explore Ash being betrayed by a new character who has ulterior motives and tries to undermine his journey.
One possible plot could be that a villain uses hypnosis on Ash Ketchum to make him do their bidding in Pokémon battles. So instead of using his own strategies, he's forced to follow the commands of the hypnotist. This would create a lot of chaos and challenges for his friends as they try to figure out what's wrong with Ash and how to break the hypnosis.
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.
Well, one possible plot could be that Ash, while on his Pokémon journey, somehow stumbles into the wizarding world. Harry Potter then takes Ash under his wing and shows him around Hogwarts. Ash could introduce his Pokémon to the students at Hogwarts, and they could all work together to solve a mystery that involves both magic and Pokémon powers. Maybe there's a dark wizard who wants to use Pokémon for evil purposes, and Harry and Ash have to stop him.
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.
Well, perhaps May decides to pursue her own Pokémon journey alone, leaving Ash behind. This could break Ash's heart as he might have developed deep feelings for her during their adventures together. Maybe she meets a new trainer who she seems more interested in, and Ash feels replaced.