A great Deku and Eri fanfiction could be set in a future where Eri has grown up a bit. She and Deku go on a mission together to rescue some hostages. Deku is proud of how far Eri has come and they work together as a great team.
The theme of family or a found family is also common. Deku and Eri can be seen as forming a sort of family unit. Deku takes on a big - brother or father - like role for Eri, and they develop a deep bond that is similar to what one would find in a family.
Some Tumblr blogs are dedicated to sharing and recommending fanfiction. You could search for My Hero Academia fanfic blogs and then look specifically for Deku and Eri stories. They might also have reviews or summaries to help you decide which ones to read.
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.
There's also 'Deku and Eri's Adventures'. In this fanfic, they go on various adventures together in the world of heroes. It's full of exciting action scenes and also some really sweet moments between the two characters.
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 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.
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.
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.