Well, obviously Deku in two forms - the younger and the older version are the main characters. The younger Deku is the one who is still growing and learning, while the older Deku has more experience.
Midoriya's friends like Uraraka or Iida might also be involved. They could be witnesses to the strange encounter between the two Dekus and might have their own reactions and contributions to the story. For instance, they could help the younger Deku process the information he gets from his older self.
Another plot might involve the older Deku being from an alternate timeline. He shows the younger Deku different ways he could have developed his skills and relationships, which makes the younger Deku reevaluate his own path as a hero.
Another possibility is that a new villain emerges as a result of the time - travel or whatever mechanism brings future Deku to the present. This villain could be from a future where Deku's actions (or inactions) have led to a more dangerous world, and he travels back in time to stop Deku from making those mistakes again. However, his methods are extreme and dangerous, making him the antagonist of the story.
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.
In 'deku meets future deku fanfiction', perhaps the future Deku warns the present one about upcoming threats. He could tell him about villains he'll face in the future and how to defeat them. This would give the present Deku an edge in his battles and also in his growth as a hero.
Obviously, Deku is one of the main characters. Since it's a Saiyan Deku fanfiction, he is the one with Saiyan powers. Other characters from My Hero Academia like All Might or Bakugo can also be important characters in the story, as they interact with Deku in different ways because of his new Saiyan abilities.
One possible plot could be that Villain Deku is on a mission to undermine the hero society and crosses paths with Hero Deku. They engage in a fierce battle where Villain Deku taunts Hero Deku about his ideals. In the end, Hero Deku manages to get through to a small part of Villain Deku's conscience, leaving him with some doubts about his villainous path.
I think Uraraka would be very excited. She has always been a big supporter of Deku. Meeting Future Deku would give her a chance to see how far he has come, and she might also get some inspiration for her own hero journey. Plus, she's a friendly and emotional character, so she'd likely be thrilled at the opportunity.
Obviously, Deku would be the main character since it's about being reborn as him. But other characters like All Might are also likely to be important. All Might was a huge influence on Deku in the original story, and in the fanfiction, he could still play a crucial role in mentoring the reborn Deku.
It's Deku and Todoroki. Deku is known for his never - give - up attitude and his ability to inspire others. Todoroki is recognized for his cool and sometimes aloof exterior but deep - down caring nature. In the fanfiction about their marriage, these traits interact and influence how they communicate, support each other, and face challenges as a couple.
Well, obviously Deku is one of the main characters. Since it's about him going berserk, he's at the center of the story.