If it was a fanfiction like that, it would completely distort the perception of Harry Potter. Harry is a symbol of hope and light in his own universe, but if he was like Darth Vader, he'd be seen as a much darker and more menacing figure. As for Darth Vader, his iconic status as a Sith Lord might be diluted if his traits were randomly imposed on Harry Potter. It would be a very odd mash - up.
In such a fanfiction, Harry Potter becoming a Darth Vader fan could lead to him adopting some of Vader's more authoritarian and decisive ways. He might start to use more forceful methods to deal with his enemies in the wizarding world, which would surely cause a stir among his friends and foes alike. For example, instead of relying solely on magic spells in a traditional way, he could be influenced by Vader's use of the Force and try to develop something similar in the magical realm.
He might become more brooding and less open. Harry has always been a bit of an open book, but Vader's influence could make him more withdrawn as he contemplates power and the dark side.
One idea could be a crossover where Darth Vader somehow ends up in the wizarding world. Maybe he's mistaken for a very powerful dark wizard and Hogwarts has to figure out how to deal with him. Another could be a story where Harry Potter discovers the Force and has to learn to use it alongside his magic. A third idea might be a scenario where Darth Vader is resurrected in the Potterverse and teams up with Voldemort, causing chaos for the Order of the Phoenix.
Well, Harry would probably grow up to be a very different wizard. Darth Vader's strict and often dark ways of discipline might make Harry more brooding. He might be more inclined to use power in a more forceful way rather than the more subtle magic in the wizarding world. He could also be more isolated as Darth Vader isn't exactly a warm and fuzzy father figure.
In such a fanfiction scenario, Harry Potter would likely gain some very unique skills. Darth Vader's training would probably introduce Harry to the dark side of the Force, which could be a huge shift from his traditional wizarding training. Harry might become more aggressive and less reliant on his wand, perhaps learning to use the Force for combat and other tasks. However, this would also create a moral conflict within him as the values of the wizarding world and the Sith teachings are quite different.
Well, it could be a wild mix of two very different universes. Maybe Harry would inherit some of Vader's powers along with his wizardry. He could have the ability to use the Force in the wizarding world, which would make him extremely powerful. He might use the Force to enhance his spell - casting or to sense danger in a different way than other wizards.
One possible plot could be that Naruto, through some strange magical or technological accident, gets transported to the Star Wars universe. There, he is influenced by the dark side of the Force and starts to take on the characteristics of Darth Vader. Maybe he has a run - in with Palpatine - like figure who manipulates him. He could start wearing Vader - like armor as a symbol of his new, darker self.
There are quite a few out there. Some fanfictions explore what would happen if Darth Vader was a teacher at Hogwarts. He would be a very strict and intimidating one for sure. His methods of teaching, which would be influenced by his Sith training, would be a huge contrast to the usual Hogwarts professors. It makes for an exciting read as it completely changes the dynamic of the Hogwarts environment.
One interesting plot twist is when Darth Vader turns out to be related to a key character in the Harry Potter universe. It completely changes the way you view both universes and how they could be connected on a deeper level.