I think Naruto and Sasuke are obvious choices. But characters like Sakura also have potential. Her growth from a somewhat naive girl to a strong kunoichi can be analyzed through her inner self. Additionally, Kakashi, with his hidden past and his own set of emotions, could be a great character to focus on in this kind of fanfiction.
Naruto himself is very likely to be a focus. His complex personality and strong emotions make him a prime candidate. Also, Sasuke, with his brooding nature and his past traumas, would be another. His internal journey of revenge and later redemption can be explored in an 'inside out' way.
In this fanfiction, Naruto is the key character as the sensei. For Team 8, Hinata is likely to be a major focus. Naruto might see a lot of his own insecurities in her and strive to help her become a stronger ninja. Kiba, with his energetic and sometimes brash nature, would provide a contrast to Naruto's more balanced approach, so he would be focused on for his development. Shino, with his unique abilities and quiet nature, would also be in the spotlight as Naruto tries to understand him better and help him integrate more with the team.
The main character is Naruto. However, there could be new characters created specifically for this fanfiction. For example, an ancient being that has been living inside the Shinju for ages. And of course, some of Naruto's mentors or enemies from the original series could also make an appearance, depending on the direction the fanfiction writer wants to take. If it's about Naruto facing a new threat inside the Shinju, an old enemy could be resurrected in a different form inside the Shinju's realm.
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.
The main characters are Naruto and Kushina. Given that it's about Kushina inside the seal within Naruto, their relationship and interactions would be at the heart of the story. It's possible that other characters from the Naruto universe could play supporting roles. For example, the Nine - Tails might also be important as it is also associated with the seal. But overall, Naruto and Kushina are the key figures in this fanfiction.
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.
Naruto and Sasuke are very common. Since they are the main characters in the original series, in fanfiction about Infinite Tsukuyomi, they often play crucial roles. Naruto might be the one trying to save everyone from the illusion, and Sasuke could be either helping or having his own internal struggle within the dream.
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.