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.
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.
Definitely The Flash. Their powers are quite different. The Flash's super - speed could be an amazing contrast to Deku's more physical - based quirk. Deku could learn about time - related concepts from The Flash, and they could have an interesting dynamic when it comes to chasing villains.
Some characters might find it interesting and enjoy seeing different takes on their adventures. But others could be skeptical or even a bit uncomfortable with how they're portrayed in fanfiction.
Toga could be an ally. Since she has her own complex relationship with the idea of heroes and villains, she might find Deku's transformation interesting. She could provide some of her unique skills, like shape - shifting, to help Deku in his villainous plans. Additionally, Twice might also join them because of his loyalty to Toga and his own sense of being an outcast in the hero world.
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.
Themes in such fanfiction usually include resolving conflicts between different versions of the Doctor and showcasing their growth through this encounter. Elements like shared memories and unique abilities coming into play are also common. Sometimes, it even explores the concept of fate and destiny for the Doctors.
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 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.
He could be getting married to Ochaco Uraraka. They have a great friendship and there are many hints in the series that suggest a deeper connection between them. Their personalities complement each other well, with Deku's determination and Ochaco's cheerfulness.