Well, if Izuku gets the Hero Killer's quirk in fanfiction, he might face a lot of internal conflict. On one hand, his original goal of being a hero is all about saving people with a smile. But the Hero Killer's quirk is more about punishing those who are 'false heroes'. Izuku would have to struggle with how to use this new power in a way that aligns with his values. Maybe he'd try to reform the quirk's nature, using it to expose real villains who are hiding among the heroes instead of attacking heroes directly.
There are also fanfictions where Izuku's quirk has a hidden aspect. Maybe his connection to One For All unlocks memories or abilities from the previous users in a more profound way. For instance, he might be able to access the combat knowledge of the past users directly, almost like having an in - built combat instructor guiding his every move.
One great Izuku fanfiction is 'A Different Path'. In this story, Izuku gets a quirk that's not One for All but something entirely unique. It shows his journey of self - discovery and how he still manages to be a hero in his own way. The author does an amazing job of exploring Izuku's character and his relationships with other characters like Bakugo and All Might in this new scenario.
It would make his hero journey much more complex. He'd have to constantly prove that he's not going to use the quirk in the wrong way. The public might be skeptical of him at first. But if he can show that he's still the same Izuku at heart, just with a new tool to fight villains, he could win them over.
In some fanfictions, because of his powerful quirk, Izuku becomes a sort of mentor figure to his classmates. For example, with his enhanced understanding of his quirk in these stories, he can help his friends train and develop their own quirks better. So his relationship with them shifts from being just a peer to a more guiding one.
The unique quirk concept. Izuku with a light quirk offers a fresh take on his character. It allows for different power - based storylines compared to his original quirk in the canon.
One idea could be that Izuku's quirk is related to time manipulation. He could use it to slow down time during battles, giving him an edge in combat. For example, he could dodge attacks with ease and plan his counter - attacks carefully.
The 'Underdog to Hero' trope is also common. Even when Izuku has a new quirk, he often starts off being underestimated. He has to prove himself not only to his classmates but also to the heroes around him. This trope is great for character development as we see Izuku grow in confidence and skill.
The theme of redemption can also be seen in some 'boku no hero izuku quirk fanfic'. Maybe Izuku made some mistakes in the past related to his quirk or how he used it. He then has to work towards making up for those mistakes and becoming a better hero. This theme adds depth to his character and makes the story more interesting as we see him grow and change.
Some characters might be immediately terrified. They could avoid Izuku or even try to drive him away. For example, classmates who don't understand his quirk might spread rumors about him.