Sakura might be one of the first to suspect. Since she's observant and has known Naruto for a long time, she could notice small details that give away Naruto's true gender. For example, Naruto's different approach to certain situations compared to the male characters.
Kakashi is a character that some fanfictions play with in terms of hidden gender. As a very popular and mysterious character, changing his gender can bring out new aspects of his personality. His relationship with his students, especially Naruto, Sasuke and Sakura, would be very different. Also, Itachi could be a character where a hidden gender aspect in fanfiction can create a whole new set of storylines and emotional arcs.
Naruto is obviously one of the main characters as the title implies it's about his hidden strength. Sasuke is also likely to be a main character as he has a long - standing relationship with Naruto. Sakura might also be important as she is part of Team 7. Kakashi could be involved too, as he is their sensei and has a role in guiding Naruto to discover his hidden strength.
Naruto, Sasuke, and Sakura are the most obvious choices. They are the main characters and their interactions are always interesting. Naruto would bring his energetic and fun - loving nature, Sasuke his brooding intensity, and Sakura her intelligence and strong - willed personality.
Naruto himself could be in jail fanfiction. Maybe he was framed for a crime he didn't commit and has to prove his innocence from within the prison. Sasuke is also a likely candidate. His brooding and sometimes anti - social nature could lead to him getting into trouble with the law in a fanfiction setting. Another one could be Orochimaru. Given his history of unethical experiments, it's not hard to imagine him ending up in jail in a fan - created scenario.
Iron Man is another candidate. Tony Stark's technology and Naruto's ninja techniques could be a great contrast. They could learn from each other. Stark might be interested in Naruto's chakra - based powers and try to replicate them in some way, while Naruto could be amazed by Stark's high - tech gadgets.
Definitely Naruto and the Punisher first. Naruto would be intrigued by the Punisher's methods and would try to understand his motives. It's also possible that Sakura could interact with the Punisher. She's a strong - willed character, and she might try to either stop the Punisher if she thinks his actions are too extreme or help him if she sees the good in his goals. And Jiraiya might also be a character who could cross paths with the Punisher. Jiraiya's worldliness and his own sense of right and wrong could lead to some deep discussions between them.
Naruto is a very likely character. Given his rebellious nature and his history of being an outcast in some ways, it's easy to imagine a scenario where he gets caught up in something that lands him in prison. His determination to break free and his loyalty to his friends would make for an interesting story.
Sasuke. He has such a complex character arc and his death would have a huge impact on the story. So, in fanfiction, he's a likely candidate to come back to life and continue his journey. Maybe he has some unfinished business or a new purpose to fulfill.
Obviously Naruto himself is very likely to appear. He is the main character after all. Sasuke would also be a common character as his relationship with Naruto is so central to the story. Then there are the important figures like the Hokages, especially the ones like Minato and Hashirama who had a big impact on Naruto's life. And of course, characters like Kakashi who played a major role in Naruto's growth.
Definitely Naruto himself (as the gender - bended version). Since the whole concept often revolves around him changing genders, he's at the center of most of these fanfictions. Another popular character would be Sasuke. His relationship with a gender - bended Naruto always makes for an interesting dynamic.