One common theme is deception. Since Kyubey is known for being manipulative in the original series, in human form he often uses his appearance to deceive others, especially when it comes to recruiting magical girls.
One interesting story could be that in human form, Kyubey is a student in a high school. He tries to secretly recruit girls for the magical girl system without them realizing his true identity at first. He befriends a group of girls who are interested in the supernatural, and as the story unfolds, they start to suspect something strange about him.
The theme of redemption can also be found in some Homura x Kyubey fanfiction. Even though Kyubey has caused a lot of harm, some stories might explore the possibility of his redemption. Maybe through Homura's influence, Kyubey starts to see the error of his ways. Or perhaps Homura realizes that Kyubey is just a product of his species' nature and tries to find a way to change that nature, which could be seen as a form of redemption for both of them in a way.
Well, identity crisis is very common. Sonic is used to being a blue hedgehog with super speed, but when he becomes human, he has to re - evaluate who he is. Adaptation to a new physical form and a different set of abilities or lack thereof is also a theme. And then there are the themes of friendship and love. His relationships with his friends might change, and he might even find new romantic interests in his human form. For example, in some fanfictions, Sonic has to learn how to communicate his feelings in a more human way, which was not necessary when he was a hedgehog.
One common theme is the struggle between her kaiju nature and her human form. For example, in many stories, she has to deal with emotions and limitations that are new to her in human form.
One interesting idea could be exploring the human version of Kyubey's complex moral compass. Since in the original, Kyubey's actions are often morally ambiguous. In a human Kyubey fanfiction, we might see how a human with similar characteristics struggles with ethical decisions. For example, a human Kyubey might have to decide whether to sacrifice a few for the greater good, just like the original character does when making magical girl contracts.
One common theme is his adjustment to the human world. Since he's from a different form originally, how he copes with human emotions, relationships, and society is often explored.
One common theme is identity. As Amaterasu takes on human form, she has to figure out who she is now. Is she still the same divine being or has becoming human changed her fundamentally?
First, you need to understand the essence of the original Kyubey character. Know his motives, actions, and the way he interacts with others. For a human Kyubey, you can then translate those traits into a human context. For example, if Kyubey is cold - hearted in the original, a human version might be emotionally detached.
One popular theme is Arceus as a human form interacting with other Pokémon in a more human - like way. For example, it could be forming friendships or rivalries with trainers. Another theme is the power dynamics. Since Arceus is a powerful Pokémon, in human form, it might struggle to control its powers or use them for the greater good. Also, a theme could be about Arceus' origin story in human form, exploring how it came to be and what its purpose is in this new form.
Romance is a huge theme. Many fanfictions focus on Sonic and Amy's relationship, exploring how their feelings develop for each other in their human forms. It could be a slow - burn romance or a more immediate attraction.