One common theme is family loyalty. Given the strong bond within the Kamado family in the original story, this is often explored further in fanfiction. Another theme could be redemption, especially when it comes to Nezuko's situation as a demon. Writers might also focus on the theme of survival, as the family has to deal with the constant threat of demons.
Demon Kamado family fanfiction can be really exciting. It could focus on the untold stories of the Kamado family. Maybe there are stories about their ancestors and how certain family traits were passed down. Some fanfictions might also create alternate universes where the family has different powers or face different types of demons. It gives fans a chance to expand on the original story and get creative with the characters and their relationships.
There should be no sex content in kids' cartoons. The industry has strict guidelines and standards to protect children from such inappropriate themes. Cartoon creators are well aware of their responsibility to provide innocent and educational entertainment for kids.
I think having a clear theme and a good understanding of the topic you want to convey are essential. Also, strong drawing skills and the ability to use humor or satire effectively can make the cartoon stand out.
The main difference might be in the themes and content. Futa docking manga often has more unconventional and adult-oriented elements that aren't common in regular manga.