Well, he might learn about his future achievements. For example, he could find out if he becomes the greatest hero. Maybe his future self gives him some advice on how to deal with upcoming villains more effectively.
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, it could be that Deku's younger self is initially in awe of his future self. They might start training together, with the older Deku showing the younger one new moves and ways to improve his skills. Also, the older Deku could tell stories about his battles and how he overcame various villains, which would give the younger Deku more confidence.
At first, it might be a bit tense. Natsu would be full of questions and his future self might be a bit reluctant to share everything. But as they start to talk, they could become more understanding of each other.
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.
One possible theme is self - discovery. The older Deku has already gone through a lot of growth, and by meeting his younger self, he can see how far he has come. The younger Deku can also start to understand his potential better.
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 this fanfiction, Steven could learn valuable lessons from his future self. For example, he might find out how to better use his powers or how to handle difficult situations with the Crystal Gems. Maybe his future self would warn him about upcoming threats and how to avoid them.
In the 'natsu meets his future self fanfiction', one possibility is that his future self warns him about certain events that are going to happen. For example, about an upcoming battle that is more dangerous than any he has faced before. His future self could also show Natsu new magic techniques that he will develop over time. It could also be a situation where Natsu's future self is not what he expected, perhaps having made some mistakes that Natsu now has the chance to avoid.
Well, perhaps Percy's future self comes back to warn him about something. It could be about an upcoming danger that he needs to avoid. Or maybe his future self has some wisdom to impart, like how to better handle his relationships with his friends or family. This encounter could change Percy's perspective on a lot of things in his current life.