Another idea is that Naruto has a dark secret regarding his past training. Maybe he had to do some unethical or dangerous things during his training with Jiraiya that he's ashamed of. This could include using forbidden jutsu or making deals with dark entities in order to gain power, which would be a huge shock to his friends and the village if it ever came out.
Another theme is the past haunting the present. In many 'dark secrets novel', something that happened long ago, like a crime or a family scandal, resurfaces and affects the current situation. It's like in 'The Secret History' where the past actions of a group of students come back to haunt them in the present day. It creates a sense of mystery and dread as the characters try to deal with these resurfacing secrets.
Betrayal is a common theme. Characters often betray each other's trust, which leads to the creation of dark secrets. For example, in a story, a friend might steal a valuable item from another and keep it a secret, causing a rift in their relationship.
The dark secrets can add a lot of tension to the plot. If Naruto has a secret related to an evil organization, it could make his friends and allies question their trust in him. They might start to suspect his every move, and Naruto would have to struggle to prove his loyalty while also dealing with the truth of his situation.
The 'Girl on the Train' can also be considered. It's full of dark secrets as the main character, who has an alcohol problem, uncovers the truth about a missing woman and the complex relationships around her. There are secrets within secrets in this story, which makes it really engaging.
Well, from the title 'Edge of Evil: A Dark Secrets Novel', it might center around a character who is gradually being drawn towards an evil state. The 'dark secrets' could be things like past traumas, family secrets, or hidden agendas. Maybe the main character discovers these secrets one by one and has to deal with the moral implications of them. For example, it could be that the secrets are related to a crime that happened long ago, and the main character is somehow involved or is trying to uncover the truth about it.
One dark secret could be a character's hidden past of having committed a crime they've never been caught for. This can add a layer of mystery and moral ambiguity to the story.
One of the dark secrets is that in some of the original fairy tales that Disney stories are based on, there are much darker and more violent elements. For example, in the original 'Snow White', the queen not only tries to poison Snow White but also demands her heart and liver as proof of her death. Disney had to tone down these elements to make the stories more suitable for a younger audience.