One possible reason could be that in the fanfiction, Ash might feel like Charizard has become too powerful and uncontrollable. Another reason could be that Ash has some new Pokémon that he wants to focus on more, and he thinks Charizard is holding him back in terms of training and strategy.
Perhaps in the fanfiction, Ash gives up on Charizard because of external pressures. Maybe there are trainers or organizations in the story that threaten to harm Charizard if Ash doesn't let it go. Or it could be that there are some unforeseen circumstances like a new law in the Pokémon world of the fanfiction that restricts Ash from keeping Charizard. This would force Ash to make a difficult decision for the sake of Charizard's safety.
In some fanfiction scenarios, Ash might give up on Charizard if there are conflicts within the Pokémon team. For example, if Charizard's presence is causing tension or jealousy among other Pokémon. Also, it could be that the fanfiction writer wants to explore a different path for Ash where he doesn't rely on Charizard's strength. This could lead to new character development for Ash, as he has to adapt to not having Charizard by his side.
Another reason might be related to Naruto's growth in the fanfiction. If Naruto becomes too powerful or changes in a way that Hinata no longer feels connected to him, she could give up. Suppose Naruto starts to embrace a more solitary and power - hungry path, which goes against Hinata's values of family and connection. This divergence in their paths could cause her to lose hope in their relationship and give up on him.
One possible reason could be a lack of time. Maybe Ash has become too busy with other aspects of life, like work or studies, and can't find the time to write fanfiction anymore.
The plot might involve Ash starting his journey all over again, but this time with a Charizard as his very first Pokémon. Since Charizard is a powerful and somewhat stubborn Pokémon, Ash would have to learn how to handle it right from the start. He could face challenges like other trainers trying to steal Charizard because of its rarity. Ash would have to prove his worth as a trainer and his connection with Charizard to keep it by his side.
The plot could be that Ash is orphaned at a young age and Charizard finds him. Charizard then decides to raise Ash in the wild. Ash grows up learning survival skills from Charizard and how to communicate with other Pokémon directly.
The plot could be that Ash enters a Kalos - only Pokémon competition with Charizard. They face strong opponents with unique Kalos Pokémon. Charizard might learn new moves by observing the local Pokémon. For example, it could see a Delphox use a powerful fire - type move and try to adapt it to its own style.
The plot might revolve around Ash leaving Charizard because he believes Charizard has a greater purpose elsewhere. For example, there could be a group of wild Charizard in need of a leader, and Ash realizes that his Charizard is the one. So he leaves it to fulfill that role. In the fanfiction, we could see Charizard initially being reluctant but then gradually accepting its new destiny and leading the group in a new and better way.
Maybe the loss of a very dear Pokémon friend. If something bad happened to a Pokémon he was extremely close to, like it getting seriously injured or lost forever, it could be so devastating that he loses the will to continue his journey and thus gives up.
It could be that in this fanfiction, Ash had some personal issues come up. For example, he might have received some bad news from his family back home that made him lose focus on training and eventually give it up. Another reason could be that he found a new passion or interest that diverted his attention away from training.
One possible reason could be the overwhelming loss he has faced. He's lost many dear friends and mentors throughout his journey. For example, when Jiraiya died, it was a huge blow to him. Another reason might be the constant pressure of being the savior of the village. The weight of everyone's expectations could lead him to feel so tired and defeated that he gives up on life.
One possible reason could be that Naruto realizes Sasuke is too far gone on his dark path. After numerous attempts to bring him back, Naruto might start to think that Sasuke doesn't want to be saved. He may see that Sasuke's actions are causing too much harm to the village and the people he loves, and Naruto has to prioritize them over Sasuke. So, he gives up on the idea of bringing Sasuke back.