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.
In some fanfictions, there could be a plotline where the early mastery of One For All causes Izuku to attract unwanted attention from villains much sooner. This forces him to step up and become a hero in a more rushed and dangerous way. He has to learn to balance his new power, school life, and the threat of villains all at once, which makes for a very engaging story.
Another plotline could involve Izuku having to quickly develop his combat skills to match the power of One For All earlier. So, he might seek out different mentors or training methods that are not part of the normal curriculum. This could lead to some interesting interactions with other characters who are either helping or hindering his progress.
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.
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.
A third plotline could be that Izuku has to fight All for One not just physically, but also in a battle of wits. All for One is a very cunning adversary, and Izuku would need to outsmart him. This could involve Izuku using misdirection, setting traps, and predicting All for One's moves in order to gain the upper hand.
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.
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 interesting izuku all for one fanfiction could be where Izuku uses the All for One power in unexpected ways to save his friends. For example, he might use it to transfer quirks temporarily to those in need during a battle, creating unique and exciting fight scenes.
One interesting story could be about Izuku exploring new powers of One For All in a different dimension. He might face challenges that test his limits and how he uses the power in unique ways.
One interesting fanfiction could be where Izuku uses the combination of All for One and One for All in unique ways to defeat powerful villains. He might use All for One to take away the quirks of enemies and then use One for All to enhance his physical abilities to an even greater extent than normal.
One possible plotline could be Izuku having superhuman abilities that are not like the normal quirks in the original story. For example, he might have the ability to manipulate time or space in ways that are completely unheard of in the My Hero Academia universe. This could lead to him being both feared and revered by others.
One possible plotline could be Izuku using his knowledge of Pokemon to help him in his hero training. For example, he might study how Pokemon use their unique abilities and apply similar strategies in combat against villains.