Well, Naruto would likely become more complex in his personality. He'd still have his core of kindness, but he'd also develop a more mysterious side. He might be more reserved in showing his emotions, like Tsunade who hid her pain behind a tough exterior. He would also be more diplomatic, as the Sannin often had to deal with different factions and villages. He'd start to use his words more effectively in addition to his fists, and his sense of humor might become more refined and less goofy as he tries to match the Sannin's sophisticated personas.
His personality could also become more calculating. Since the Sannin were known for their strategic thinking, Naruto would start to think several steps ahead. He might be more secretive about his plans too. He would no longer be the open - book Naruto. Instead, he would be like Jiraiya, who had his own secrets and plans. He would also show more patience, which is something he lacked before but the Sannin had in abundance when dealing with complex situations.
He might become more serene. Since Sadaiam might have a calmer and more nature - centered philosophy, Naruto could adopt some of that. He'd be less impulsive in his actions and more contemplative.
He would become more responsible. As Hokage, he has to think about the whole village, not just his friends. So he might be a bit more serious, but still keep his positive and energetic side.
His relationships would be quite different. With Sasuke, he might be more of a mentor - like figure early on, as he'd have more knowledge and power from the Sannin's teachings. His relationship with Sakura could be more of an equal partnership in combat, since he'd have more medical knowledge from Tsunade. And with Kakashi, he'd probably still respect him, but he'd also have his own unique insights from the Sannin that could make their interactions more like exchanges between equals.
In this fanfic, Naruto's relationships would be in flux. Regarding his friends, they would look up to him as he gains knowledge and power from the Sannin. His relationship with Sakura could change as he becomes more capable of handling Sasuke - related issues. With the village elders, Naruto might gain more respect as they see his potential growing under the Sannin's tutelage. His connection with the Nine - Tails would also be different. As he learns from the Sannin, he could use the Nine - Tails' power in new ways, which might change how he is perceived by the fox and how he interacts with it. All in all, his relationships would be reshaped by the new experiences and skills he acquires from the three Sannin.
If Naruto becomes involved with Slenderman in a fanfiction, his usually cheerful and determined personality might become darker. He could be more withdrawn as he grapples with the strange and often terrifying aspects of Slenderman's world.
His personality might become more refined. Instead of being the rowdy Naruto we know, he could be more composed and polite due to the influence of the daimyo's court.
Naruto might become more aggressive at first. The influence of Venom could bring out his darker side. But as he gets used to it, he would learn to balance it with his usual positive and determined self.
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.
Naruto might become more reserved. Growing up under Zeltrech, he could be exposed to a more sophisticated and perhaps less rowdy environment compared to Konoha. So instead of his usual loud and brash self, he could develop a more calm and calculated side.
Well, first of all, Naruto's usually carefree and positive personality could be tempered with Ace's more complex emotional side. Ace had a lot of baggage related to his family and his identity as a pirate. So Naruto might start to have some internal struggles about his place in the world. He could become more reserved in some ways, like not being as quick to share his feelings. At the same time, his loyalty would become even more intense. He would be willing to go to greater lengths for his friends and the village, almost to the point of self - sacrifice, which was a big part of Ace's character. And his view of power might change too. Instead of just seeing it as a means to protect and be recognized, he could see it as a way to uphold a certain code, like Ace's code among the pirates.