Sasuke could potentially be an ally. Given their complex relationship, Sasuke might understand Naruto's situation and choose to side with him. They have a deep bond, and Sasuke has also been an outcast in a way. Another could be Jiraiya. If he were still alive, he might believe in Naruto's innocence and help him from the shadows.
Gaara could be an ally. Gaara has his own experiences of being an outcast in the past. He might empathize with Naruto and offer his assistance. Also, some of the minor characters who have always been on the fringes of the village and have a sense of justice might join Naruto. For example, those who have seen the corruption within the village first - hand.
Jiraiya might also side with Naruto. Jiraiya has always been a mentor and a father figure to Naruto. He would believe in Naruto's innocence and help him. He could use his connections and knowledge to keep Naruto safe and help him clear his name.
Kakashi could also end up being an ally. Despite his loyalty to the village, if he discovers the real reasons behind Naruto's departure and realizes that the village has made a mistake, he might choose to help Naruto secretly. He has a deep bond with Naruto and would not want to see him in harm's way.
Sasuke could potentially be an ally. Since they have a complex relationship, Sasuke might understand Naruto's situation and choose to side with him against the village. Also, characters like Jiraiya could still believe in Naruto and help him from the shadows.
In a 'naruto becomes a missing nin fanfiction', perhaps Naruto discovers some dark secrets about the village leadership. These secrets are so dangerous that the village wants to silence him. He then decides to go rogue to expose these secrets to the world. As a missing nin, he might form alliances with other outcasts who also have a grudge against the current system.
Perhaps the missing nin discovers new abilities within the Rinnegan that no one else has. They might be able to combine different paths in unexpected ways. For instance, using the Animal Path to summon creatures and then enhancing those creatures with the power of the Asura Path. This would make their summoned beasts more powerful and difficult to defeat in combat.
If Naruto and Temari were missing nin in a fanfiction, it could start with a misunderstanding. Naruto might be framed for a crime he didn't commit and Temari, believing in his innocence, decides to help him escape. They then have to live on the run. Along the way, they meet other outcasts and form a sort of rebel group. Naruto's ability to make friends easily and Temari's strong leadership skills could be the driving force behind this new group's growth.
Kimimaro could be an interesting missing nin character to explore in fanfiction. He was a tool for Orochimaru, but had his own internal struggles. You could write about his life before fully becoming a missing nin, his loyalty to Orochimaru despite his own deteriorating health, and what might have happened if he had broken free from Orochimaru's control. Another character could be Nagato. Before he became Pain, he had experiences that could be considered as leading towards a missing - nin - like existence. His views on peace and his use of the Akatsuki could be delved into in the context of a missing nin story.
One possible fanfiction could be about Sasuke becoming a missing nin again. Naruto and Sakura, now more powerful, set out to find him. They face many challenges on their journey, like dealing with other villages that want Sasuke for their own purposes. Naruto still believes in saving Sasuke from the darkness that led him to become a missing nin in the first place.
A great fanfiction idea could be that Sakura becomes a missing nin after an incident in the village. Naruto and Sasuke, who have their own issues, must now work together to find her. Along the journey, they realize how much they need Sakura in their lives and also discover some secrets about the village that made Sakura leave. This would be a story of self - discovery and friendship.
In such a fanfic, it could be that Naruto discovers some dark secrets about the village leadership. These secrets are so dangerous that the leaders try to silence him. Naruto refuses to be silenced and decides to break away from the village, becoming a missing nin. He then travels the ninja world, gathering allies who also believe in exposing these secrets and fighting against the unjust system of the village.