One common theme is betrayal. For example, Naruto might feel betrayed by the village or his friends, which leads him to go dark. Another theme is power corruption. If Naruto gains too much power too quickly, it could corrupt him and send him down a dark path.
One popular 'naruto goes dark fanfic' is 'The Darkening of Naruto'. It shows Naruto taking a more cynical path due to the harsh treatment he received in the village. Another could be 'Naruto's Shadowed Path' where he is influenced by dark forces and his personality changes drastically.
Often, in such a 'naruto goes very dark fanfic', a possible theme is the corruption of power. Naruto could be depicted as being influenced by power in a negative way, leading to a downward spiral in his character. Also, the theme of betrayal might be present, where friends turn on each other in this darker version of the story.
It could be an evil version of Sasuke. In the original story, Sasuke had his dark moments, and in a very dark fanfic, he could be fully consumed by evil and oppose Naruto in a much more malevolent way.
Well, he might cause chaos. He's got that dark power from the Naruto universe. At Hogwarts, he could be seen as a very dangerous outsider. His skills and powers would be quite different from the magic there. Maybe he'd try to find a way to use his ninja skills to gain an edge among the wizards and witches. He could also be intrigued by the magic system and try to combine it with his own powers in some strange way.
It could be about Naruto's adventures in a Western - style setting. Maybe he uses his ninja skills in a world similar to the Wild West, facing new enemies and challenges.
Naruto could also interact with Weiss. There might be some initial clashes due to their different personalities. Weiss is more refined and Naruto is more brash. But over time, they could come to respect each other's strengths. He might also teach her some ninja stealth techniques.
Naruto might meet some young and talented Kumo ninjas. They could be eager to test their skills against Naruto or learn from him. Maybe there are some unique characters like a Kumo ninja with a special bloodline limit that Naruto has never seen before.
In the 'Naruto goes to the North' fanfic, he may come across a group of nomadic ninjas. These ninjas have a different way of life compared to the ones in Konoha. They might be more in tune with nature and have unique abilities related to the natural elements of the North. Naruto could learn a great deal from them and they could also be potential allies in his adventures in the North.