One of the common themes is trust. Ran has to trust that Shinichi will come back to her, and Shinichi has to trust Ran with his secrets. Also, the theme of sacrifice comes up. For example, Shinichi might sacrifice his own safety or time to protect Ran. And of course, the theme of reunion is also there, as they are constantly in a state of being apart and then coming back together.
One common theme is the exploration of their hidden love. Since Shinichi is often in his Conan form, there's a lot of unspoken tension and longing between them in these fanfictions. Another theme is teamwork in solving mysteries. They often work together to crack cases, and this shows their strong bond.
One great Detective Conan Ran x Shinichi fan fiction is 'The Unspoken Bond'. It beautifully portrays the relationship between Ran and Shinichi, with lots of sweet and tense moments. Another is 'A New Beginning for Them' which takes a unique spin on their story and how they might finally get together.
One common theme is friendship. Their bond as detectives and friends is often explored, showing how they rely on each other, trust each other, and grow together. For example, in many stories, they will share their deductions and work together to solve difficult cases that neither could solve alone.
Rivalry is a very common theme. Kaito Kid and Shinichi are both highly intelligent characters, so their battles of wits are often central to the stories. For example, in many fics, Kaito Kid will plan a heist and Shinichi will try to stop him, creating a tense and exciting dynamic.
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.
In a fanfiction, perhaps Shinichi and Ran are on a school trip. They get separated from the group and find themselves in a beautiful, secluded spot. Shinichi looks at Ran, realizes how much he loves her, and leans in for a gentle kiss. This simple yet profound moment could be a turning point in their relationship, with Ran finally getting the affection she has been longing for from Shinichi all this time.
One common plotline is that Shinichi finally reveals his identity to Ran completely and then they kiss. This is a big moment because in the original series, Shinichi's identity is a huge mystery for Ran most of the time.
I also like 'Ran and Shinichi: Beyond the Mystery'. This fanfiction not only focuses on their relationship but also on how they grow as individuals. It shows Ran becoming more independent and Shinichi learning to balance his detective work with his personal life, all while their relationship blossoms.
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.