I'm not sure if it's popular. There are so many novels in the isekai genre that it's hard to say without more information. It could be a niche novel that has a dedicated following, or it might be relatively unknown.
I'm not entirely sure as I haven't read this novel specifically. But generally, in the isekai genre, it might be about a character being transported to another world and magic being involved in some way, perhaps the magic in this world is 'late' or different in some sense as the title 'okureteru' might imply.
I'm not entirely sure as I haven't read it specifically. But generally, 'isekai' novels often involve a character being transported to another world, and'mahou' means magic. So it might be about magic in an isekai setting that is somehow 'okureteru' which could mean 'delayed' or something similar in the context of the story's magic system.
One of the main themes could be the concept of the magical girl system itself. It might explore how the system is structured and what it means for the girls involved.
Often in such stories, there are supporting characters like the friends or family members of the magical girls. They may not have magical powers but can influence the magical girls' decisions and emotional states. For example, a best friend from school might notice the strange behavior of the magical girl and try to figure out what's going on, adding an extra layer of complexity to the story.
You can try checking some well - known online bookstores like Amazon or Barnes & Noble. They often have a wide selection of light novels, including some from the isekai genre.
The character of Aku could be developed in a 'pregnant Aku in Samurai Jack fanfiction' by exploring how he interacts with the world around him differently. He may become more reclusive as he doesn't want others to see his 'weak' state. Or he could become more aggressive in trying to protect his pregnancy, which would be a new aspect of his character. Also, his relationship with his own power might change. Maybe he has to find new ways to use his magic that don't harm the unborn, which would force him to grow and adapt in ways he hasn't before.
One idea could be a crossover where Aku accidentally opens a portal to the Naruto universe. Naruto, being the curious and brave ninja he is, decides to explore Aku's world. They could start off as enemies, but gradually come to understand each other's motives and end up teaming up against a greater evil that threatens both of their worlds.