Yes, there might be. The world of fanfiction is vast and diverse. With the popularity of both Harry Potter and the concept of Ryuji Jakkan (assuming it's a known or created concept), it's very likely that some fans have combined them in their creative works. However, it may not be as mainstream as some other Harry Potter fanfictions.
The Yakuza games story portrays Yakuza culture in several ways. It shows their hierarchical structure. The higher - ups have great power and respect, and the lower - level members have to follow strict rules. Also, their sense of justice within their own world is depicted. They have their own way of dealing with disputes and traitors which is part of their unique culture.
One common theme could be power and its consequences. Since Percy is an elder god, he has a great deal of power, and the fanfiction might explore how he uses or misuses it.
One of the best yakuza stories is about loyalty within the yakuza clans. For example, there are tales of yakuza members who would sacrifice themselves to protect their bosses. They consider honor and loyalty above all else, and these stories often show how they adhere to a strict code of conduct even in the face of danger.
One of the well - known best yakuza novels is 'Tokyo Underworld' by Robert Whiting. It delves deep into the seedy underworld of the yakuza in Tokyo, showing their power struggles, criminal activities, and the complex web of relationships within the organization. Another great one could be 'Out' by Natsuo Kirino. Although not strictly a traditional yakuza - centric novel, it involves elements of the criminal underworld where the yakuza operate, and it's a very gripping story of women involved in a crime related to the yakuza world. Then there's 'Six Four' by Hideo Yokoyama which also touches on the yakuza influence in Japanese society among other complex themes.