Well, one possibility could be that Naruto is exposed to some extremely powerful and corrupting jutsu that slowly drives him insane. Maybe he starts seeing hallucinations of his friends turning against him and that messes with his mind big time.
There's also a chance that a new, unknown character could be the main antagonist. This character could be from a far - off land with a vendetta against the Hidden Leaf. They target Naruto specifically and use some ancient and evil method to drive him crazy. Since Naruto is so important to the Leaf, this would create a lot of chaos and make the new character the main source of trouble for Naruto and the village.
No. Slavery is an immoral and unethical institution. Naruto, as a character from a series that promotes positive values like perseverance and justice, should not be associated with slavery in any way. Fanfics that involve such concepts can be considered offensive and against basic moral principles.
Well, he might encounter the Nightsisters. Maybe he'll have to learn their unique magic or face their trials. He could also form an unlikely alliance with them against a common enemy.
This is a rather dark and unethical concept for a fanfic. Naruto is a character associated with values like friendship and freedom. However, if such a fanfic were to exist, perhaps it could involve some sort of dark magic or curse that forces Naruto into a situation where he has to deal with slaves. But this goes against the original spirit of the Naruto series.
He could meet a local superhero. They might initially clash because they have different ways of doing things, but then they realize they can learn from each other. For example, Naruto could teach the superhero about chakra control and the superhero could teach Naruto about modern technology and fighting strategies used in America.
One possible plot is that Naruto arrives in America and is initially confused by the different culture. But then he starts to adapt. He could use his chakra to perform amazing feats at a local circus or fair, becoming a local sensation. He might also encounter some villains who are after his ninja powers and he has to fight them off while trying to keep his identity a secret.
Maybe Naruto is sent to Africa on a mission by the Hidden Leaf Village. Once there, he discovers a new world full of different cultures and challenges. He could encounter magical creatures or powers that are unique to Africa in this fanfic. And he has to use his wits and ninja powers to either protect these newfound elements or to stop an evil force that is threatening both Africa and his own village back in Naruto's world.
One possible plot could be Naruto exploring the modern technology on Earth. He'd be amazed by things like cars and phones which are completely different from what he has in his own world. He might use his ninja skills in unexpected ways, like using his speed to compete in track races and becoming a local celebrity.