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.
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.
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.
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.
A simple idea could be that Haru and Eri participate in a Digimon tournament together. They train their Digimon side by side, and in the process, they start to understand each other's training methods better. Their Digimon also form a closer bond as they spar with other Digimon in the tournament.
A fanfiction might focus on Haru and Eri getting lost in a strange part of the digital realm. They have to find their way back home while also helping some baby Digimon along the way. During this journey, they learn more about each other's dreams and fears. Haru realizes Eri's determination is much stronger than he thought, and Eri sees Haru's kind heart in a new light.
His friends could come to the rescue. Bakugo, for instance, might use his explosive power to break into Toga's hideout. He's always been competitive with Deku, but he also cares about him deeply.
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.