Yes, there are likely fanfictions out there with this concept. In the Naruto fandom, there are many creative interpretations. Some authors might explore Naruto having S - class monster - like powers or actually being a monster in a unique AU (Alternate Universe). It could involve him having extraordinary strength, unique abilities, or a form that is considered monstrous yet still retains his Naruto - like qualities.
In a fanfiction with Naruto as an S - class monster, perhaps the plot could start with Naruto suddenly realizing he has these strange new powers and abilities. He initially struggles to understand them and hides them from his friends. As the story progresses, he is forced to reveal his secret when a powerful enemy attacks the village and he's the only one who can stop it. However, his S - class monster form comes with a price, like slowly losing his humanity. His friends then have to find a way to save him while he battles this internal and external threat.
Sakura could be one of his allies. She could develop her own magical medical skills to support Naruto in battles. Her intelligence and combat skills from the ninja world would translate well into the mage world.
His leadership skills. Naruto is often shown to be a great leader in fanfiction. He can lead teams and villages effectively, which is a characteristic of an S - class ninja. Also, his combat abilities are enhanced. He may be able to defeat powerful enemies with ease, using new techniques and strategies. Moreover, his influence in the ninja world is expanded. He might be respected and feared by many other ninja clans, which all contribute to him being considered an S - class ninja in fanfiction.
There was no clear mention of the level of the S grade monsters in the Devour Space. Therefore, I don't know the exact level of an S-grade monster that devours the stars.
Power - up is a common theme. Since it's about S - class ninjas, the characters, usually Naruto, often gain extraordinary powers. Another theme is facing greater challenges. S - class ninjas are expected to deal with more dangerous foes and situations.
Sasuke is often involved. His brooding nature and his own complex relationship with the village could easily be tweaked in a fanfiction to make him be wrongly perceived as an S - class criminal. Naruto himself might also be involved, either as the one trying to bring the criminal to justice or as someone who believes in the criminal's innocence.
Perhaps a story where an S - class criminal in the 'naruto' fanfiction has a change of heart. They start off as a villain but due to certain events, like meeting a young orphan or seeing the destruction they've caused, they decide to turn their life around. It could involve them trying to make amends with the people they've wronged, which would be a great character - development arc.
Sure. 'A Legend Reborn: S - Class Ninja Naruto' is a great one. It has an interesting plot where Naruto quickly rises to become an S - class ninja with unique powers and adventures.