First, collect a lot of Hikaru Hayashi's manga works. Then, practice imitating their drawing steps, especially the way they handle details and create a unique atmosphere. With consistent practice and observation, you'll gradually be able to draw in a similar style.
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.
First, study Hikaru Hayashi's style to understand the proportions and features of the pretty gals. Then, practice sketching basic body shapes and facial expressions.
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.
In this type of fanfiction, the theme of power and its implications is likely. Yuichiro's dominance represents a certain kind of power. How he wields this power in relation to Mikaela and the world around them can be a theme. Another theme could be the exploration of boundaries. Both characters may need to figure out the boundaries within their relationship due to Yuichiro's dominance. And finally, the theme of sacrifice might come into play. Yuichiro might be more likely to sacrifice for Mikaela because of his dominant position, and vice versa.
You can try fanfiction websites like Archive of Our Own (AO3). There are often a large number of fanfictions about various pairings, including muichiro x yuichiro. Another option is FanFiction.net, but the content quality may vary. You can also check out some dedicated anime or manga fan communities where fans might share their own written fanfictions or recommend good ones.