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.
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.
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.
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.
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.
Obviously, Villain Deku would be the main antagonist. Since he has turned to the dark side, his actions and motives would be in direct opposition to Jirou and the values of the hero world she represents.
A new villain could emerge who is specifically targeting adult heroes. Since Deku has now become an adult, he becomes a prime target. This villain might have a grudge against the hero society's adult leadership and see Deku as a symbol of that.
Maybe in this fanfiction, Class 1 - A is on a training mission when they accidentally stumble into a time - warp and meet Future Deku. Future Deku could be a super - powerful hero, and he starts to train them in advanced techniques that they haven't learned yet. He might also share some of his future experiences with them, like the battles he has won and the villains he has faced, which would inspire Class 1 - A to strive harder in their hero training.
It could be a rogue Saiyan from an alternate timeline who has come to their universe to prove his supremacy. He is jealous of the power that Goten and Future Trunks possess and wants to take it for himself.
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.
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.