In this fanfiction, Deku's character development could be quite complex. Going back in time, he may initially be overconfident because he thinks he knows what's going to happen. But as he realizes that changing the past has unforeseen consequences, he becomes more cautious. He also might develop a deeper understanding of his friends and their potential, as he gets to see them at an earlier stage. He could start mentoring some of them, which would in turn make him more responsible and mature as a character.
Maybe in the 'manga deku goes back in time fanfiction', Deku goes back to the time before he got his quirk. He then tries to understand the origin of quirks better, using his knowledge from the future. This could lead to him developing his quirk in a completely different way, perhaps even a more powerful way. He might also use this time - travel opportunity to befriend some characters who he didn't have a chance to in the original timeline.
Well, one interesting plot point could be Deku using his knowledge of future events to prevent certain tragedies. For example, he might stop a major villain attack before it even starts, saving many lives. Another could be his interactions with past versions of his friends and how he has to be careful not to change things too much.
Villain Deku might start to question his villainous ways. Meeting Hero Deku could make him see the good in heroism. Hero Deku, on the other hand, could become more empathetic towards those who turn to villainy. He might realize that there are reasons behind it and not all villains are just 'evil' for the sake of it.
Deku would likely become more confident in his abilities. In the One Punch Man universe, he'd see Saitama's nonchalant attitude towards fighting and might start to adopt a similar mindset. He'd also have to adapt to the different types of villains. Since One Punch Man villains are often very strange and powerful in unique ways, Deku would need to be more creative with his strategies.
In addition to the main characters, there could be some minor characters from the manga. For instance, the teachers at UA like Aizawa. Aizawa might notice that something is different about Deku and start to investigate. There could also be villains from the past that Deku has to face again, but this time with his future knowledge. Characters like Stain could be interesting to include as Deku might try to change the way he dealt with Stain in the original timeline.
Often in fanfiction, Deku's power development is tied to unique training regimes. He might train with characters who have different quirks that complement his own. This could lead to him unlocking new aspects of One For All. Say, he trains with a character who can manipulate energy, and this helps him better understand and control the energy within One For All, thus evolving his power.
Deku's character development in this scenario is complex. At first, he may be embarrassed and scared by the things he can't help but say. But as time goes on, he learns to embrace this new state. He realizes that being truthful all the time, even about his weaknesses, can be a strength. His relationships evolve as his friends come to appreciate his honesty. He also grows in terms of his hero identity. Instead of trying to project a perfect image, he shows his true, flawed self, which makes him a more believable and inspiring hero in the fanfiction world.
The relationship could develop slowly. Deku would likely admire Tsuyu from afar at the beginning. Then, they could be involved in group activities where they start to work together more closely. Deku would notice Tsuyu's kindness towards others and her determination. Tsuyu, in turn, would see Deku's growth and his unwavering spirit. As they get to know each other better, their relationship would progress from acquaintance to friendship and then to something more romantic.
Deku might be initially shocked. He's used to Bakugou's aggression, but an actual injury is different. He could be in pain and disbelief at first. Then, he might try to downplay it, especially if he doesn't want to cause trouble for Bakugou, because Deku is generally a very forgiving character.
Going back in time gives Severus the opportunity to develop a different relationship with power. In the original timeline, his pursuit of power through the Dark Arts and association with Voldemort led to many problems. But if he goes back, he could use his knowledge to gain power in a more positive way. He could become a powerful force for good in the wizarding world, using his intelligence and skills to promote peace and cooperation. His character would then be seen as a reformed and wiser version of himself, rather than the conflicted and often tragic figure he was in the original story.