Another plot could involve a detective in the series, perhaps a side character, who is pregnant and uses her unique situation to solve a case. For example, her pregnancy hormones might make her more perceptive in some ways or she could use the fact that people underestimate her because she's pregnant to her advantage in uncovering the truth.
One idea could be a fanfic where Conan has to solve a mystery within a virtual reality game world. Another could be a story where he teams up with a new character who has a unique detective skill that complements his own. Maybe a fanfic about Conan traveling back in time to solve a mystery related to his parents' past.
A more complex plot could be that there is a secret connection between the two worlds. A powerful artifact from the wizarding world has been stolen and ends up in the hands of a criminal in Detective Conan's world. This leads to a race between the wizards and Conan's team to retrieve it. Along the way, they discover that there are ancient prophecies that tie the two worlds together and that the fate of both depends on getting the artifact back. This could involve a lot of character interactions, like Hermione and Ran becoming friends and sharing their knowledge, or Ron and Conan bonding over their love for food while also trying to solve the mystery.
Well, it depends on the writer's creativity. Some possible plots could be that Conan was drugged in a way that made him regress to a more infant - like state where he needed to be in diapers. Or maybe it's a time - travel - related story where he goes back to his baby days for some mission and has to deal with the diaper situation while still having his detective mind. But again, this is all speculation as I haven't read this particular fanfic.
One idea could be to write a fan fiction where Conan has to solve a mystery in an ancient, haunted castle. The castle could have secret passages and hidden rooms filled with clues that lead to a long - lost treasure. Another idea is to create a story where Conan teams up with a new character who has a unique skill, like being able to communicate with animals, which helps them solve the case in a different way. You could also write a fan fiction that takes place in a different time period, like the future, and Conan has to adapt his detective skills to new technologies.
One idea could be creating an OC who is a master of disguise, like Conan, but with a unique backstory. Maybe they are from a different country and have their own reasons for getting involved in the detective work in Japan. Another idea is an OC that has a special gadget - something that hasn't been seen in the original series yet, like a device that can analyze poisons instantly. And a third idea could be an OC who is a childhood friend of Shinichi's that has some hidden talents related to solving mysteries that come to light after his transformation into Conan.
One idea could be a crossover with Sherlock Holmes. Conan could meet Holmes in London and they could solve a complex mystery together. Holmes' deductive skills and Conan's modern knowledge would make for an exciting combination.
A great plot might be that there's a mystery surrounding a rare and valuable magical artifact. Kaito is interested in it for its magical significance, while Shinichi is on the case because it's related to a theft. They initially suspect each other but then realize they need to work together. They discover that a shadowy figure is using the artifact's legend to cover up a more sinister plot.
One idea could be focusing on the emotional journey of the character. She might feel a mix of overwhelming joy and intense fear. Another could be about the practical preparations for the arrival of four babies, like setting up a huge nursery. Or perhaps the story could revolve around how the family and friends react to this big news.
A possible plot for a 'Ranma 1/2' pregnant fanfic is that it's a result of some magical mishap. This could lead to a search for a way to reverse it, but during that time, Ranma and Akane (or whoever is involved) start to bond in new ways. They could learn more about each other's fears and hopes for the future. Or perhaps it's a plot where an enemy discovers the pregnancy and tries to use it against them, forcing them to come up with creative ways to protect themselves and the unborn child.
Well, they could face a situation where they have to go into hiding during the pregnancy because of the dangerous nature of the clone world. They might find a secluded place where Cosima can have a peaceful pregnancy, but of course, trouble still finds them. Maybe some old enemies track them down, and they have to use their wits and the help of a few trusted friends from the Clone Club to stay safe.