No, it's not appropriate. Fanfiction should focus on positive aspects such as character development, new adventures, or exploring relationships in a non - explicit way. Sexualizing a character in fanfiction goes against the spirit of most fandoms which are often family - friendly or at least should be respectful of all ages.
You could write about Killua's friendships. For example, his relationship with Gon and how they support each other through their adventures. This would be a positive and in - line - with - the - spirit - of - the - story type of fanfiction.
I firmly oppose the idea of creating or promoting fanfiction with sexual content related to Killua. Such content violates moral and ethical boundaries. The 'Hunter x Hunter' world has so many great aspects to explore in a proper way, like the complex power systems, the different cultures in the story, and the relationships between characters that are not sexual in nature.
Gon in fanfiction can be very single - minded in his pursuit of goals, just like in the original. He might be a bit more naive at times, which can lead to interesting plot twists. Killua is often portrayed as having a complex emotional side. He struggles with his past as an assassin and his loyalty to Gon. His cool exterior may crack in some fanfictions, showing his true feelings.
Yes, there are likely some fanfictions out there with such a concept. Fanfiction writers often come up with creative crossovers. You can try searching on popular fanfiction websites like Archive of Our Own or Fanfiction.net.
I think Midoriya could be his best friend. Midoriya is kind - hearted and always looking to learn, which would appeal to Killua. They could bond over their shared desire to become stronger and protect others.
Often, they show a deeper level of commitment. The mpreg forces them to rely on each other more. For example, Gon might become more protective of Killua during the pregnancy.
As I mentioned before, 'lemon' implies adult content which is not suitable for discussion. So, I can't really talk about the characteristics of such fanfics.