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.
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.
As of now, Deku is very much alive in the manga. The plot hasn't reached a point where he meets such a fate. Usually, main characters like Deku have significant story arcs and aren't easily killed off.
The main differences are in their expressions and some minor details of personality. In the manga, Deku might seem a bit more intense, while in the anime, he's sometimes presented a bit softer.
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.
In the manga, Deku experiences various ups and downs. He not only becomes stronger physically but also matures emotionally as he takes on greater responsibilities. There are also many unexpected plot twists that shape his journey.