One common theme is power exploration. Since both BNHA and BSD have characters with unique abilities, fanfics often explore how these powers interact and combine. For example, a character's quirk from BNHA might be used in an unexpected way when paired with a power from BSD.
There's also 'Jujutsu and Bungo in an Alternate Realm'. This fanfic delves deep into how the powers from both universes would interact. It shows the characters trying to understand each other's abilities and form alliances. It's a long read but definitely worth it for fans of both series.
Most BSD light novels often have complex characters. Since they are based on literary figures, they bring in the depth and complexity of those original personas. For example, a character might have the same traits as a famous writer but with a unique twist for the BSD universe.
It could be Atsushi. He is a central character in BSD and his kind - hearted yet troubled nature would make for an interesting exploration in a story about a spring without someone important. Maybe the person missing is someone who has been like a mentor to him.
Well, there's a 'bnha x bsd fanfic' named 'Allied Powers'. In this fic, the author does a great job of making the two different worlds interact. It shows how the characters from My Hero Academia and Bungo Stray Dogs might work together, fight together, and even form new relationships. It's full of action and some really cool crossovers of their powers.