Harry Potter himself is a likely candidate. Given his heroic nature and the need to constantly fight against dark forces, an Akuma no Mi could be a great addition to his arsenal of skills. It would give him new ways to protect his friends and defeat his enemies.
Power dynamics is a common theme. Yujiro is often depicted as extremely powerful, and the relationship with the OC can be shaped by this. For example, the OC may be in awe of his strength or try to prove themselves equal to him.
When writing a Harry Potter and Yujiro Hanma fanfiction, it's important to establish the rules of the combined world early on. Will Yujiro be able to use magic? If not, how will he defend himself against magical attacks? You can also play with the idea of cultural differences. Yujiro comes from a very different kind of world compared to the wizarding world. This can lead to some interesting misunderstandings and comical situations. For instance, Yujiro might think the Sorting Hat is some kind of strange creature trying to attack him and try to fight it off. As you write, keep the pacing right, not too slow or too fast, and make sure the story has a satisfying ending, whether it's a happy ending or a more bittersweet one.
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.
Well, the Akuma origin story could be related to ancient curses. Maybe it was a result of a forbidden ritual gone wrong. In certain tales, Akuma is seen as a manifestation of the darkest human emotions. It gains its power from fear, anger, and hatred, and over time, it has evolved into a terrifying force that preys on the weak and the vulnerable.
I'm not sure if there is a specific fanfic exactly like that. There are so many Naruto fanfics out there, and while some might involve Naruto having an interest in Akuma in some creative crossovers, it would take some serious searching through fanfic platforms to find one.
Well, akumas typically have a back story related to a source of evil or a corrupting influence. It could start with a character who has some sort of inner turmoil or a tragic past. This internal struggle makes them vulnerable to the influence that turns them into an akuma. For example, if a character has lost a loved one and is filled with grief and anger, that might be the trigger for the akuma transformation, leading to a whole new set of actions and a role in the story as an antagonist.
Another place is FanFiction.net. There are some really good and unique 'Akuma no Riddle' fanfics there. Just use the search bar and type in 'Akuma no Riddle' to find relevant stories.