The more intelligent Mavericks, like Sigma, might see this as an opportunity. They could try to use X's time - traveling to their advantage. For example, they might try to manipulate the past X into making a mistake that would benefit them in the future. Or they could try to learn about future technology from X by interrogating him.
However, some enemies might also be afraid. If they know that X has come from the future, they might fear that he has some new power or knowledge that could defeat them easily. So they might either run away or become more desperate in their attacks against X, which could lead to different battle outcomes compared to the original timeline.
Some of the Mavericks might be very confused at first. They'd see Megaman X in a time where he wasn't supposed to be. This could throw off their plans as they try to figure out what's going on.
He could also gain a new perspective on his own development. By seeing his past self, he might understand how he became the hero he is in the present. He could pass on some of his present - day knowledge to his past self, like better combat techniques against the Mavericks. This would potentially change the course of his own history in interesting ways.
Issei could be a bit nervous at first. He might worry about what his classmates will think. But if they show positive reactions, he could become more confident and start sharing more about his writing. However, if they react negatively, he may feel a bit down and might even regret that they saw it.
Voldemort and his Death Eaters might see it as a sign of weakness or a strange Muggle - influenced fad. They could mock Harry for it, thinking that he's straying from his pure wizarding roots. However, this could also be a miscalculation on their part as Harry might use their underestimation to his advantage.
If Sasuke travels to the past, his relationships would be completely transformed. With his clan members, he would have a chance to interact with them in a new light. He might try to warn them about their impending doom, which could change the dynamic from one of normal family relations to one of him being a seer of sorts. Regarding Kakashi, Sasuke could either use his knowledge of Kakashi's teaching methods to his advantage or try to change how Kakashi trained him. And when it comes to his enemies in the past, like Orochimaru, Sasuke might approach him differently. He could try to avoid Orochimaru's influence altogether or try to turn Orochimaru into an ally against the greater threats that he now knows about from the future.
His powers would likely shock the past characters. They might see him as a great hope or a potential threat. If he uses his powers openly, it could change the way the Z - fighters train and view their own power limits.
Sasuke would probably be a bit dismissive at first. He'd think Naruto was being childish, but deep down he might be a little curious. Sakura would likely be more understanding. She might encourage Naruto to share more about Megaman, thinking it's a harmless interest. Shikamaru, on the other hand, would just roll his eyes and say it's too troublesome for him to care about Naruto's new fandom.
Well, his powers would also attract a lot of attention. Scientists in the past might be intrigued by his abilities and try to study him. This could either lead to a better understanding of superhuman abilities in that timeline or it could create chaos as different groups try to capture him for their own purposes. His presence could also inspire others to strive for greater physical abilities.
Darth Vader would likely be conflicted. On one hand, he could see the potential in his younger self to become a powerful Sith. On the other hand, he might be reminded of his own lost innocence. He might give his younger self some warnings about the path he is about to take, but also encourage the dark journey.
When Boruto goes back in time, say to the era of the Third Hokage. His relationship with the village elders and the teachers at the ninja academy would be completely new. He would be seen as an oddity since he has knowledge and skills that are not from that time. He might form unexpected friendships with some of the young ninjas of that era. His relationship with his own family members in the past, like his grandparents, would also be very interesting. He could learn a lot about his family's history and values from them, which would change how he views his family in his own time.
If Sauron time travels, he might become more desperate. For example, if he travels to a time when his power is waning, he'll be desperate to regain his former glory. This could make him take more risks than he usually would.