Well, they might be shocked at how much technology has advanced in the future depicted in the fanfiction. Maybe there are new types of elemental powers or different ways of training ninjas. It could also be that the relationships between characters have changed in unexpected ways, and they would have to come to terms with that.
Well, 'Nana Shimura: Unseen Tales' is worth checking out. It's a fanfic that fills in the gaps in her story. It has detailed descriptions of her daily life and her struggles. The author does a great job of making her seem like a real, relatable person rather than just a character from a story.
You can start by looking on fanfiction websites like Archive of Our Own. They usually have a wide range of fanfictions, including ones about Nana Shimura.
They might gain a new perspective on their own abilities. If the future fanfiction shows ninjas with enhanced powers, the past characters could start to think about how they could develop their skills further. For example, if in the future they can combine elemental powers in new ways, the past ninjas might start experimenting.
Such comics are likely to contain explicit and potentially harmful content. It's best to stay away from them and focus on more positive and appropriate forms of entertainment.
Another aspect could be a story where Nana Shimura somehow survives and has to face the new world of heroes. How would she react to the current state of hero society? She might be shocked by the commercialization of heroism and try to bring back the purer form of being a hero that she knew.
One possible plot could be that Izuku, in his journey to become a great hero, discovers some hidden knowledge or power left behind by Nana Shimura. He might find an old diary or a secret training ground that was once used by her. This encounter could give him new insights and inspiration for his hero training.
A common theme is her relationship with her students or proteges. Since she was an important figure in the 'Boku no Hero' world, many fanfics explore how she influenced the younger heroes.