One interesting Yujiro Hanma fanfiction could be about his early days of training. It might explore how he developed his extreme strength and fighting skills. Maybe it shows his encounters with various opponents during his youth and how he overcame them to become the fearsome fighter he is known as.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fan - created works from various fandoms, including Yujiro Hanma fanfiction. Just use the search function and type in 'Yujiro Hanma' to see what's available.
A crossover with 'One Punch Man' would also be interesting. Yujiro Hanma might initially see Saitama as a weakling due to his unassuming appearance. But as the story unfolds, he realizes Saitama's true power. It could lead to some really comical and action - packed scenes as they interact.
In a crossover with 'Bleach', Ichigo would be a great character to pair with Yujiro Hanma. Ichigo has his own brand of badassery with his Hollow powers and Zanpakuto skills. The contrast between his more spiritual powers and Yujiro's brute physical strength would create an interesting dynamic.
One great story could be where the OC is a strong - willed martial artist who challenges Yujiro Hanma. Their interactions are filled with intense battles and mutual respect that gradually turns into a complex relationship.
First, understand the character of Yujiro Hanma. Know his personality traits like his extreme strength and pride. Then, create your OC with a distinct personality. Start with an interesting encounter between them, like a chance meeting at a fighting tournament.
One common theme is friendship. Despite coming from different backgrounds, Naruto and Hanma often form a bond in these fanfictions. Another theme could be power - struggle, as they compare and combine their different powers.