Zero Two is a great character to focus on. Her complex backstory, her unique personality, and her relationship with Hiro offer a lot of material for a fanfic. You could explore her past more deeply, like her experiences as a half - klaxosaur, half - human and how that has shaped her into the character we see in the series.
One idea could be a 'what if' scenario where the characters from 'darling in the franxx' are transported to a modern - day Earth. They would have to adapt to a world without the threats they faced in their original setting and deal with things like modern technology and social norms.
Another theme is self - discovery. The reader, who is inserted into this strange and complex world of Darling in the Franxx, often has to figure out their place in it. They learn about their own strengths and weaknesses while interacting with the pre - existing characters and the unique situations in the franxx world. This theme of self - discovery can be really inspiring as it shows the growth of the reader - inserted character throughout the fanfic.
The main themes in 'Darling in the Franxx' include self - worth. The characters struggle with their place in society and how they are valued. Also, friendship and loyalty play a big part as the characters fight together and rely on each other. And of course, the theme of growing up and maturing is quite prominent as they face various challenges.
The anime 'Darling in the Franxx' is not for all ages. It delves into themes like self - discovery, relationships, and there are also some action - packed and intense mecha fight scenes. Additionally, the way the characters' relationships develop, especially the romantic aspects, makes it more suitable for an older audience, say, teenagers and above.