One aspect to consider in this fanfiction could be the power dynamics. If Yuichiro is dominant, it might lead to interesting character interactions with Mikaela. For example, Yuichiro could be the one making decisions in their adventures together, while Mikaela might play a more supportive role. It also adds an element of tension and drama to their relationship, as the dominant - submissive dynamic can create various scenarios for character growth and conflict.
One common theme is the deep emotional bond between them. Given their relationship in the original work, fanfictions often explore how they support and care for each other on a more profound level. Another theme could be adventure, where they go on quests together and their relationship develops during those experiences.
I'm not that familiar with 'yuichiro hyakyua' crossover fanfiction, but often in crossovers, themes like friendship between characters from different worlds can be common. Maybe also the exploration of different powers or abilities when the characters interact.
Survival is also a big theme. When he is in a crossover with a more dangerous world, like a post - apocalyptic or a world full of powerful enemies, Yuichiro has to use his skills and wits to stay alive. It creates a lot of tense and exciting moments in the fanfiction.
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.
I'm not sure specifically about 'yuichiro hyakyua' crossover fanfiction. You could try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net. They usually have a wide range of crossover fanfics.