His ability to move would change drastically. Vader has his slow, yet intimidating, mechanical walk. Naruto might adopt a similar gait as he becomes more like Vader. In terms of combat, Naruto's Rasengan could be combined with the dark side of the Force. It could become a more destructive and corrupting attack, maybe with a black, swirling energy around it. Also, his ability to sense others, which was based on chakra before, could now be a more Force - like sense, allowing him to detect enemies even through walls and over long distances.
His relationship with Sasuke would become extremely complicated. Instead of being rivals - friends, Sasuke would see Naruto as a threat. He might try to fight Naruto to bring back the old Naruto or to stop his tyranny. Sakura would be horrified at Naruto's transformation. She might initially try to reason with him, but if that fails, she could become one of his opponents. His relationship with his former sensei Kakashi would also be strained as Kakashi would not recognize the Naruto who has become Darth Vader.
Naruto's ability to use chakra and Darth Vader's connection to the Force could potentially create a very powerful combination. For example, Naruto could use his chakra to enhance Vader's lightsaber strikes, making them even more deadly. Or Vader could use the Force to help Naruto better control his Nine - Tails chakra.
His personality could become darker. Darth Vader's influence might make Naruto more brooding and less of the optimistic character he is. He could start to value power more than friendship at times.
In the 'naruto is like darth vader fanfiction', their journey of self - discovery could be seen as similar. Naruto has to constantly figure out his place in the ninja world and how to control his power. Darth Vader is on a journey of self - discovery too, as he grapples with his identity as Anakin Skywalker and his role as Darth Vader. They also have a sense of isolation in some ways. Naruto often feels alone in his struggles, and Darth Vader is isolated in his dark armor, cut off from his former self and many human connections.
His personality might become more brooding. With Darth Vader's influence, Naruto could lose some of his naivete and become more cautious.
Darth Vader could emerge victorious. His mastery of the Force gives him an array of abilities such as telekinesis and mind control. He could potentially use these to disable Naruto before he has a chance to launch a major attack. Also, Vader's lightsaber skills are top - notch. If he can get close to Naruto, he could deliver a fatal blow. And in a one - on - one situation, without the help of his friends, Naruto might find it difficult to counter Vader's unique powers.
Naruto's shadow clone jutsu could have a new twist. Instead of just creating clones for battle, the clones could be used to perform tasks around the One Punch Man universe, like helping to rebuild after a villain attack. And his ability to use nature energy could be combined with Saitama - like strength, allowing him to deliver punches that not only have physical force but also some kind of energy - based impact.
If Ahsoka joins Vader in fanfiction, their relationship would become extremely complex. At first, it would likely be filled with anger and resentment from both sides. Vader might see her as a reminder of his past failure as Anakin, and Ahsoka would see him as the fallen Anakin she once knew.
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.
One way this could happen is through some sort of interdimensional rift. Maybe a powerful jutsu in the Naruto universe accidentally tears open a portal to the Star Wars galaxy. Naruto, being the curious and brave ninja he is, gets sucked into it. Once in the Star Wars universe, he's exposed to the dark side of the Force, which gradually corrupts him and he starts to take on the persona of Darth Vader. He might even learn some of the Sith ways from Vader's own teachings if he somehow travels back in time to Vader's era.