Well, one possible plotline could be Izuku's struggle to control the sudden influx of power from One For All at an earlier stage than in the original story. He might face challenges in keeping it a secret from his classmates and teachers, adding a layer of secrecy and tension.
Obviously, Izuku Midoriya is the main character as he is the one mastering One For All early. Katsuki Bakugo might also be prominently involved. Since he has a long - standing rivalry with Izuku, his reaction to Izuku getting such a powerful quirk early on could be a major part of the story. Maybe he feels jealous or challenged in a new way.
Obviously, Izuku is the main character. He's the one who is mastering One For All. All Might also often appears as he was the previous user and is Izuku's mentor in many fanfictions. Bakugo can also be a significant character as he serves as both a rival and sometimes an unexpected ally to Izuku in his journey with One For All.
One great story could be 'Izuku's Ascent'. In it, Izuku really hones his skills with One For All in unique ways. He has to face challenges that force him to understand the full potential of the quirk quickly. Another is 'Mastering One For All: Izuku's Journey' where he trains under an old mentor who knew the previous users. This mentor guides him to unlock hidden aspects of One For All.
Well, there's a fanfic called 'Early One for All'. It shows Izuku getting One for All in his early days at U.A. High. The story dives deep into how he has to hide this new power from some villains who are already targeting him because they sense the change in the power dynamics. It's really exciting as it adds a new layer of danger and secrecy to his character arc. There's also 'Swift Inheritance' where the early gain of One for All makes Izuku question his own worthiness more intensely as he feels he hasn't proven himself enough yet.
In many BNHA fanfictions where Izuku meets All Might early, it often changes the entire trajectory of Izuku's development. For example, he might gain confidence much sooner. Early meetings can also lead to a more in - depth mentorship that might deviate from the original storyline. This could result in Izuku getting special training earlier and potentially unlocking his Quirk in a different way.
One consequence could be a faster growth in Izuku's power. With All Might's early guidance, Izuku may master his Quirk more quickly. Another is that his relationships with other characters might change. For example, Bakugo could become more jealous or, on the contrary, might be inspired to work harder.
Izuku's character would likely be very different. He might be more cynical at first. Since he's raised by All for One, he would be exposed to a lot of villainous thinking. But as he grows, his inborn sense of justice could start to resurface and cause internal conflict.
You can try websites like Archive of Our Own (AO3). It has a large collection of fanfiction for various fandoms, including My Hero Academia where Izuku and these powers are from. Just use the search function and type in relevant keywords like 'Izuku' 'All for One' 'One for All' to find the fanfictions you're interested in.