Well, one key element is the transformation into human forms. It often leads to exploring how they adapt to human characteristics like emotions, relationships, and social norms. For example, in some fanfics, Mikey might struggle with the concept of human privacy while Leatherhead could be fascinated by human technology.
There are definitely some out there. However, the quality can vary greatly. Some might focus more on their friendship with a hint of something more, while others could be full - on adventure stories that also explore their relationship in a different way. It really depends on what you're looking for in a fanfiction.
One great Mikey and Leatherhead human fanfiction is 'The Bond Beyond Species'. It shows an interesting take on their relationship in a human - like world, with lots of adventure and character development.
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.
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.
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.