Mikey might be a character created to tell a story about Pop Rocks. He could be a fictional child used to illustrate the appeal or novelty of these candies. For example, he could be a curious and adventurous little boy who wasn't afraid to try new things, like Pop Rocks, which were quite different from other candies at the time.
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.
In many fanfictions, Mikey is the optimist. He looks at the world with a positive outlook and tries to find the good in Rahzar. His friendly nature makes it easier for him to approach Rahzar, even though Rahzar might seem intimidating at first. Rahzar is usually depicted as having a lot of strength and power. He's a force to be reckoned with, but his character can also be complex, with hidden emotions that Mikey helps him uncover.
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.
April and Casey might also be part of this fanfiction. April could use her reporting skills to try and figure out what's going on with Mikey's mutation, and Casey would be ready to fight any new threats that come along because of Mikey's changed state. Also, Splinter, as their sensei, would be guiding them all through this difficult time, providing wisdom and training to help Mikey control his new powers if he has any.
One possible plot could be a crossover where Yujiro Hanma somehow finds his way into the wizarding world of Harry Potter. Maybe he's on a quest for ultimate power and hears rumors of the powerful magic there. He could start challenging the most powerful wizards, like Dumbledore, thinking their magic is just another form of strength to be conquered.