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.
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.
One common theme is friendship. Their meeting often focuses on how they support and encourage each other. Another theme could be growth, both in terms of their quirk development and personal growth.
Friendship might be a theme. The older Naruto could emphasize to his younger self the importance of his friends, like Sasuke, Sakura, and Kakashi. Also, the theme of destiny. The older Naruto can tell his younger self that he has a great destiny ahead and should keep striving for it. And it could be about forgiveness too, as Naruto has had to forgive many of his enemies throughout his journey.
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.
Another theme could be about isolation. Ruby might feel alone in her problems, and self - harm could be a way for her to cope. In the RWBY universe, she has her friends, but the fanfic could be delving into how she might feel that they don't truly understand what she's going through, leading her to this self - destructive behavior.
One possible theme could be betrayal. Maybe Uraraka does something that makes Deku feel betrayed, like siding with an enemy or revealing one of his secrets. Another theme might be jealousy. If Uraraka is jealous of Deku's power or relationships, it could lead to her hurting him in the story.
In this fanfiction, Percy might be shocked at first to see his younger self. He could offer advice to his younger version, like how to better handle the challenges in the magical world. Maybe he'd warn his younger self about certain enemies or mistakes he made in the past.
In a fanfiction scenario where Naruto meets his younger self, it would be a very unique situation. Naruto might show his younger self some of the new jutsus he's mastered, like the Rasengan variations. His younger self would be eager to learn from him. Also, Naruto could talk to his younger self about his dreams and how he's still holding on to them, even after all the difficulties. This meeting could potentially change the course of his younger self's life, as he could be more determined to reach his goals faster with the knowledge of his future self.
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.