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.
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.
The main differences are in their expressions and some minor details of personality. In the manga, Deku might seem a bit more intense, while in the anime, he's sometimes presented a bit softer.