In some fanfiction concepts, Naruto might have a hidden mission that he can't disclose to Sakura. His actions in pursuit of this mission, like working with her enemies without her knowledge, could be seen as betrayal. This could be part of a much larger plan to save the village or even the entire ninja world, but Sakura doesn't understand it at first.
One possible scenario could be Naruto being forced into a situation where he has to seem to betray Sakura to protect her from a greater danger. For example, he might be blackmailed by an enemy into staying away from her or acting against her interests.
Maybe in the fanfiction, Naruto is under the influence of some dark magic or mind - control that makes him turn on Sakura. It could be that an evil character has manipulated Naruto's thoughts and actions, leading him to do things that Sakura perceives as betrayal.
In some fanfictions, Naruto might punish Sakura for betraying their friendship. For example, he could distance himself from her, making her realize how her actions have hurt him.
One possible scenario could be a character being betrayed by a teammate. For example, Ruby might be betrayed by Weiss if Weiss is lured to the dark side by some powerful and evil force. This could lead to intense battles and emotional turmoil for Ruby.
One possible scenario could be that Sakura is feeling down because she failed a difficult medical ninjutsu exam. Naruto, being the good friend he is, takes her out for her favorite ramen and tells her stories of his own numerous failures and how he never gave up. This cheers her up as she realizes that failure is just a stepping stone to success.
Well, in this type of fanfiction, it might be that Twilight Sparkle is betrayed by a mentor figure. This mentor could have been teaching her false information all along, causing her to make mistakes that put her and her friends in danger. Or it could be that she is betrayed in a love - related situation. For example, if she has feelings for a certain pony and that pony pretends to reciprocate only to use her for something else, like getting access to the library in Canterlot. There are so many directions this fanfiction could take.
Well, it could be that Draco betrays Hermione's academic trust. Since they are in a world where magic and knowledge are highly valued, he might steal her research or ideas for a spell or a magical discovery. This would be a huge betrayal as Hermione values her intellectual pursuits greatly. Also, he could betray her trust regarding a secret mission they are on together. He might abandon her in a dangerous situation, like in a dark forest full of dangerous magical creatures or in an old, enchanted castle filled with dark magic traps, just to save his own skin.
Perhaps in the fanfiction, Danny is betrayed by a new ally he trusted. This new character could have joined Danny in his fight against evil ghosts, but in reality, they were working for a more powerful and evil ghost all along. They might use Danny's trust to learn about his weaknesses and then expose him to danger, leaving Danny feeling hurt and alone, having to reevaluate who he can trust in the future.
Perhaps Bell Cranel is betrayed by someone he trusts in a love interest situation within the fanfiction. For example, his romantic partner might be using him for their own gain and then betrays him when they get what they want.
In Camp fanfiction, Percy could be betrayed when the camp leaders are deceived by false evidence against him. Let's say an enemy creates some fake proof that Percy has been colluding with their side. The leaders, without fully investigating, might turn on Percy. Also, there could be a love - related betrayal. If Percy has a romantic interest in the camp, the person he likes could be forced by their family or some external pressure to break Percy's trust and use his feelings against him, for example, to make him give up his position in the camp or some important secrets.
One possible scenario could be that Ash betrays May for a powerful Pokemon. Maybe he gets offered a super rare and strong Pokemon but has to cut ties with May to get it. Another could be that May betrays Ash because she is influenced by a rival trainer who convinces her that Ash is holding her back from reaching her true potential as a Pokemon trainer.