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.
Spiderman has his super strength, agility, and the famous spider - sense. In comparison, MHA characters have a wide variety of quirks. For example, Todoroki has his ice and fire powers. Spiderman's agility might be on par with some of the more acrobatic MHA characters like Sero. But his spider - sense gives him an edge in combat that most MHA characters don't have. However, quirks like All Might's super strength can rival Spiderman's own strength.
One aspect of Mha Tomura x Himiko fanfiction could be the exploration of their unique relationship dynamics. Tomura's complex character, with his goals and his own brand of villainy, and Himiko's rather eccentric and blood - loving nature can create some really interesting interactions. Writers might delve into how they understand each other on a deeper level, perhaps sharing a common view of the world that is different from the heroes' perspective.
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.
No, it's not appropriate. Deku is a character with his own depth, goals, and story within the MHA universe. Fanfic should aim to explore his hero journey, his relationships in terms of mutual respect and support, not create content that focuses on inappropriate or disrespectful ideas like this.