With the teachers, they would have to treat Izuku differently. They might give him more individual attention and training. For example, Aizawa could be more hands - on in helping Izuku control his quirk better. As for villains, they might see Izuku as an easier target at first, but then realize he's a force to be reckoned with on his own.
With All Might, Izuku's relationship could shift. All Might would have to guide Izuku in a different way since Tatsumaki powers are not like One For All. He would need to learn new ways to train Izuku and also make sure Izuku doesn't lose his original heroic spirit while using these new powers.
His relationship with Bakugo would be very different. Instead of Bakugo looking down on Izuku for being quirkless, he might be jealous of Izuku's powerful fire quirk. This could lead to even more intense rivalries between them. With All Might, All Might might be more cautious in training Izuku as a fire quirk could be more dangerous to handle without proper control.
With his classmates, some might be scared of Izuku. They've known him as a kind - hearted person, but the All for One quirk has such a dark reputation. However, some like Ochaco might try to understand him better and support him through his journey with this new quirk. His relationship with the teachers would also be under scrutiny. They would need to ensure that Izuku doesn't use the quirk for evil purposes, which could lead to a lot of extra monitoring and a sense of unease in their interactions.
His relationship with All Might would become extremely complicated. All Might, who passed on One For All, would be shocked and perhaps a bit disappointed that Izuku now has the All For One quirk. He might question his decision to choose Izuku as his successor. With Bakugo, their rivalry could take a different turn. Bakugo might be both envious and afraid of Izuku's new power, leading to more intense confrontations. And with his friends like Iida and Ochaco, they might be initially hesitant around Izuku, not knowing if he can still be trusted with such a villainous - associated quirk.
His relationship with All Might would be very different. Instead of being a successor, he might become more of a protege in a general sense. All Might could still mentor him, but their bond wouldn't be centered around One for All.
His relationship with All Might would also be affected. All Might might be more cautious about Izuku's quirk as it's very powerful and could potentially disrupt the fabric of time. Izuku, on the other hand, could use his quirk to go back and learn more about All Might's past, which could strengthen their bond as he gains a deeper understanding of his mentor. However, it could also create a rift if Izuku discovers something in the past that he doesn't like about All Might's actions.
His relationships would be turned upside down. His friends would be heartbroken and might have to re - introduce themselves to him. Some might become overprotective, while others could be distant, not knowing how to handle this new Percy.
In certain fanfictions, the izuku system is a secret. This secrecy can strain his relationships as he has to constantly lie or be evasive about his sudden improvements. But when the truth comes out, it can either lead to a huge rift or a deeper understanding among his friends, depending on how the writer chooses to portray it.
His relationships with his friends would likely become more complex. If he starts using his time quirk to change things, his friends might not remember the original timeline, but Izuku would. This could create a sense of isolation for him as he holds memories that no one else does.
With Sasuke, their rivalry might take on a new dimension. Sasuke could be both intrigued and threatened by Naruto's robotic abilities. He might see Naruto as an even greater obstacle to his own goals, or he could potentially try to use Naruto's robotic side for his own purposes.