He could meet an alternate version of himself. This Naruto might have made different choices in life, like choosing not to pursue being a Hokage. It would be interesting to see how they interact.
Perhaps in this alternate reality, there are no ninja villages at all. People live in a more unified world, but they face a new and powerful enemy that Naruto has to help fight. He would have to adapt his skills and learn new ways of combat that are unique to this world.
He might meet alternate versions of his friends. For example, Sasuke could be a completely different person with different goals and a different personality. Maybe he's a peaceful scholar instead of a revenge - seeking ninja.
His chakra might function differently. In the alternate reality, the rules governing chakra could be altered. For instance, his Nine - Tails chakra could have a different color or effect.
In the alternate dimension, Naruto could find that the Akatsuki are actually the protectors of the villages. This would completely change his view of the world. He would have to re - evaluate his ideas about good and evil and decide whether to help them or oppose them depending on their true motives.
Well, in some 'naruto goes to alternate universe fanfiction' plots, Naruto might find himself in a world where all the ninja clans have merged into one big super - clan. He'd have to navigate the complex politics within this new structure. Also, there could be a plot where he enters an alternate universe where the Fourth Great Ninja War never happened, but there are new threats emerging that he has to face. And then there's the idea of a universe where Naruto's parents are alive, and he has to deal with a completely different family dynamic.
Perhaps he'd meet a different Hokage. One who has led the village in a completely different way than in his own dimension. Naruto could learn a great deal from this Hokage about leadership and different ways to protect the village.
Sasuke is often a common character. Since they have such a deep relationship in the original Naruto story, it's likely they'll interact in the alternate universe too. Kakashi could also be a common one. He was Naruto's teacher, so it makes sense for him to be around in different universes. And Sakura, as part of the original team, is also likely to interact with Naruto in these fanfictions.
Sasuke is often still very popular in alternate reality fanfiction. His cool and brooding nature can be explored in different settings. Naruto himself is also a common favorite as writers love to put him in new and strange situations. And Kakashi is popular too, his mysterious past and badass skills are always interesting to play around with in different realities.
One interesting plot could be that Naruto is born into a different clan, like the Uchiha. This would completely change his story arc as he'd have to deal with the Sharingan and the Uchiha's dark past. Another idea is if all the characters' genders were swapped. It would bring a fresh perspective on their relationships and how they interact in the ninja world.