Perhaps in the fanfic, Ash betrays Tyson because of a misunderstanding. There could be a false rumor spread about Tyson being involved in some unethical Pokémon training methods. Ash, believing the rumor without fully investigating, turns his back on Tyson. Later, when he realizes the truth, he has to work hard to regain Tyson's trust.
Another idea is that Ash is influenced by an evil organization. They convince him that Tyson is a threat to their plans. So, Ash betrays Tyson by revealing his training secrets or leading enemies to him. But as the story progresses, Ash breaks free from the organization's control and tries to make amends with Tyson.
One possible plotline could be that Ash is influenced by a powerful and evil force. This force convinces Ash that Cynthia is somehow a threat to his ultimate goal, like becoming the greatest Pokémon trainer. So, he betrays her by stealing a crucial Pokémon of hers during a big tournament. This would lead to a lot of drama and conflict as Cynthia would be shocked and heartbroken, and Ash would start to realize the error of his ways as the evil force begins to control him more.
One possible plotline could be that Akeno is forced to betray Naruto due to some external threat or blackmail. Maybe an enemy has something over her that she values greatly, like the safety of her friends or family in her own world. So, she has to act against Naruto to protect them.
One possible plotline could be that Naruto is somehow transported to the MLP world. He might be tricked into believing the ponies are a threat and thus 'betray' them by taking actions against them. For example, he could be misled by a dark magic user in that world who convinces him the ponies are hoarding power that could be dangerous.
Another plotline might be that Percy betrays God out of a sense of self - preservation. Suppose there's a great threat coming, and Percy believes that siding with an enemy of God is the only way to save himself or the world he cares about. He might think that God's power is not enough to protect them, and so he makes a deal with the other side, which is seen as a betrayal.
One possible plotline could be that Harry betrays Draco in a duel. Maybe there was some sort of misunderstanding or a secret deal that Harry made with someone else that required him to turn on Draco during the duel. For example, if there was a dark wizard threatening Harry's friends and the only way to save them was to sacrifice Draco in the duel by using a forbidden spell against him.
One possible plotline could be that Ash and his Mega Charizard embark on a journey to a new, uncharted region. There, they face a powerful new villain with a team of never - before - seen Pokémon. Along the way, Ash has to train Charizard even further to unlock new Mega Evolutions. They meet new friends who help them in their battles and exploration.
A plot could involve the Leaf Village betraying Naruto when a powerful and ancient curse is placed on him that makes him seem like a mind - controlled puppet of a great evil. The village, in its panic, decides to take preemptive action against Naruto without fully investigating. This could lead to Naruto having to fight not only the external forces that are trying to use the situation to their advantage but also his own former comrades from the Leaf Village who are now hunting him down.
Perhaps Harry discovers a dark secret about the side he has been fighting for. This discovery makes him feel betrayed and he turns to Tom thinking that Tom offers a different kind of solution. It could be that Tom presents himself as a reformed person or has a convincing argument about a new order in the wizarding world that Harry, in his disillusioned state, starts to believe. Then, the story could follow how Harry's relationships with his old friends deteriorate as he gets closer to Tom's side.
It could also be due to some external influence. Perhaps an evil organization has manipulated Ash in some way, forcing him to act against Alola. They might have used mind control or blackmailed him with something he holds dear, like the safety of his Pokémon.
Perhaps the plot could involve Sakura having to prove her loyalty again. Naruto may be initially skeptical of her change of heart. So, Sakura has to go through a series of trials. For example, she might have to sacrifice something important to protect Naruto or the village, which would be a way to show that she truly regrets her past betrayal.