Naruto might become more sly. Since Loki is a trickster, Naruto could pick up some of his devious ways. He may not be as straightforward as he used to be.
His personality could change quite a bit. Loki is known for his cunning and manipulative nature. Naruto, under Loki's training, might start to view situations from a more strategic and less impulsive perspective. He could become more calculating and less likely to rush into things without thinking. However, he might also lose some of his pure - hearted naivete that makes him so endearing in the original series.
His personality might become more brooding. With Darth Vader's influence, Naruto could lose some of his naivete and become more cautious.
His personality might change in several ways. He would probably be more humble as he realizes the vastness of the power he is being exposed to. Also, he could become more of a mentor - like figure to the younger ninjas in Konoha. Since he is getting trained by such a powerful entity, he would have a greater sense of responsibility and would want to pass on his knowledge. He might also become more open - minded, as the Grand Priest's teachings could expose him to new ways of thinking and different cultures if the fanfiction has elements of different universes interacting.
His confidence could take on a different form. While Naruto has always been confident in his abilities, training with Zoro would make him more self - assured in a more reserved way. Zoro's tough training would force Naruto to prove himself not just to others but also to himself, and this could lead to a more mature kind of confidence. Also, Naruto might become more introverted as he focuses more on his training, similar to how Zoro is often focused on his own improvement.
Naruto might become more calm and collected. Goku has a certain simplicity and focus in his personality. Naruto could pick up on that and not be as impulsive as he usually is.
He might become more cautious. Tsunade is a more calculating ninja compared to some of Naruto's previous mentors. So Naruto could start to think before he leaps, rather than just charging into battle head - first all the time.
Well, Naruto might become more disciplined. Since Zoro is very strict in his training regime, Naruto would have to follow rules and be more punctual. He might also become more confident in a different way. Instead of relying on his usual loud - mouthed bravado, he would gain confidence from his new skills learned from Zoro. His sense of loyalty might also be enhanced, as he would respect Zoro as his teacher and protector.
He may develop a more calculating side. Black Zetsu is all about manipulation. Naruto could start thinking several steps ahead in a more devious way. For example, he might start using people's weaknesses against them, which is completely against his original nature. This change in personality would likely cause a rift between him and his friends who know him as the always - honest and straightforward Naruto.
His confidence might become more of an overbearing arrogance. Since Acnologia is so powerful, Natsu could start to think he's invincible after being trained by him. He might become more solitary too, as Acnologia is often depicted as a lone figure. Natsu might start to distance himself from his Fairy Tail friends because he feels he's on a different level now.
His confidence might increase even more. Female Madara is a strong figure, and being trained by her would give Naruto a sense of having access to greater power. So he would be more self - assured in his actions.
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.