The Phantom Troupe could be antagonists. They are a group of dangerous criminals in the Hunter x Hunter world, and they might see Kaito as a threat or a target for their own purposes. Another potential antagonist could be some of the more corrupt members of the Hunter Association who might not like Kaito's independent ways. There's also the possibility of new, original antagonists created specifically for the fanfiction, like a powerful dark magician who wants to use Kaito's power for his own evil plans.
A possible plot for 'Kaito Hunter x Hunter fanfiction' is that Kaito enters a special tournament in the Hunter x Hunter world. This tournament has rules and challenges that are different from the normal ones. He meets new and interesting characters during the tournament, some of whom become his friends and some his enemies. As he progresses through the tournament, he uncovers a conspiracy that is related to the very foundation of the Hunter x Hunter world. He then has to decide whether to expose it and risk his life or keep quiet and let the conspiracy continue.
For Miku, she could be kind - hearted, just like the Cinderella we know. She might be shy at first but has a strong will inside. She is creative, maybe shown through her music or art. Kaito could be brave, like Romeo, not afraid to pursue Miku despite the potential difficulties. He is also charming and confident, which makes him attractive to Miku.
One great 'kaito x shuichi fanfiction' is 'The Unseen Bond'. It has a really interesting plot that delves deep into their relationship. The characterizations are on point, making it a very engaging read.
Their relationship could develop in many ways. For example, Kaito might become more responsible and caring towards Shinichi during the pregnancy. He could start to see Shinichi in a new light, not just as a rival but as someone he truly loves and wants to protect. Shinichi, on the other hand, might become more reliant on Kaito, which could strengthen their bond.
One great Kaito Kid x Aoko fanfiction is 'The Hidden Feelings'. It delves into the unspoken emotions between them. It has some really sweet moments where Kaito's charm and Aoko's feisty nature play off each other in an endearing way.
Love is also a prevalent theme, whether it's a budding romantic love or a deep platonic love. Fanfic writers explore different aspects of their relationship, from the initial attraction to the development of a more profound connection. Some stories may show them falling in love slowly over time as they spend more time together through their musical activities.
One common theme is the clash of their personalities. Hakuba is logical and analytical, while Kaito is more flamboyant and mysterious. Another theme is the idea of trust, as they often start as adversaries and have to learn to trust each other. And mystery - solving is also a big theme, as they are both involved in mysterious situations in the original stories.
You could try looking on Archive of Our Own (AO3). There are often many well - written Vocaloid fanfics there. Just search for 'Kaito x Gakupo' and you'll likely find some interesting ones.