Perhaps in the fanfic, Akeno is influenced by a powerful and evil entity. This entity manipulates her mind, making her see Naruto as an enemy. It could be a dark magic or some sort of mind - control device. As she betrays Naruto, she starts to struggle within herself as her true self tries to break free from the influence.
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.
One possible plotline could be that Naruto somehow ends up in Akeno's world. They meet and at first Akeno is a bit shy around Naruto. Naruto, with his positive attitude, gradually befriends her. They could go on adventures together, like facing some common enemies in Akeno's realm. Naruto's ninja skills would be very useful in such situations.
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.
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 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.
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 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.
In a Naruto and Akeno love fanfiction, it could start with Akeno being curious about Naruto's unique powers. She approaches him to learn more. Naruto, being the friendly person he is, starts to teach her. As they train together, they share their dreams and fears. Naruto dreams of a world at peace, and Akeno wants to find her true self. Their conversations deepen their bond. Over time, they realize that they have fallen in love. Maybe there are some obstacles like the disapproval of their respective friends at first, but they overcome it all in the end.
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.