It depends on your preferences. Some might like a comic for its unique style and story, while others might prefer x comic for its specific themes or characters.
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.