Typically, in 'ran x shinichi fanfiction', the focus is on their relationship. It often explores the unspoken feelings between them. For example, there are fictions where Shinichi's secret identity as Conan creates a lot of tension in their relationship.
Adventure themes can also be found. Writers may put Kudo and Ran in new and exciting adventures that are different from the original story. This gives a fresh take on their characters and how they interact. Another common theme is mystery, as both characters can be associated with solving mysteries in their own ways in the original, so fanfics might expand on that aspect in new and creative ways.
A frequently seen theme is the idea of a shared destiny. Fanfic writers often emphasize how Conan and Ran are meant to be together despite all the obstacles in their way. The concept of trust is also common, as Ran has to trust Conan even when she doesn't fully understand his actions sometimes. And of course, there's the theme of their love story, which is often told in a more elaborate and sometimes alternate way compared to the original anime.
Romance is a big theme as well. Writers explore the relationship between Conan and Ran in different ways, from the angst of Conan hiding his true identity while still having feelings for Ran, to more fluffy and sweet relationship developments.
One common theme is the mystery of their relationship. Since Shinichi has his detective work that often takes him away, there's always a sense of mystery about when he'll be fully present in Ran's life in a lot of fanfictions.
I'm not sure specifically which 'kudo x ran fanfic' to recommend. You could try searching on fanfiction websites like Archive of Our Own. There are often a lot of fan - created stories there.
One common theme is the exploration of their relationship's development. Since they have a complex connection in the original work, fanfictions often delve deeper into how their feelings for each other grow over time.
FanFiction.net is also a good place to look for 'Ran x Shinichi fanfic'. There are many different versions of their relationship explored in the fanfics on this site. You can search by the characters' names or relevant tags.
One popular 'Ran x Shinichi fanfic' might be 'A Love Rediscovered'. It beautifully portrays their relationship with lots of emotional moments. Another could be 'Ran's Wait and Shinichi's Return' which focuses on the longing and the re - connection between them.
One key element is the emotional journey. Shinichi and Ran have to deal with a whole new set of emotions. Ran may feel a mix of joy, fear, and excitement. Shinichi, on the other hand, has to step up and be more responsible. Another element is how their relationship evolves. They become closer as they are about to start a family. Also, the reactions of their friends and family play an important role. Their friends might be surprised, happy, or worried for them.