Often, these fanfics might explore the relationship between rindou and eishi in different scenarios. It could be about their adventures together, or how they support each other in difficult situations.
Another option is to check out some specialized anime or manga fanfic websites. Some of these sites have a large collection of fan - created works. You may need to do a bit of digging around, but it's possible to find some great rindou x eishi fanfics that way.
Another option is to check out Tumblr. There are often fanfiction recs (recommendations) on Tumblr blogs dedicated to Food Wars. You can follow these blogs and see if they've shared any Soma x Rindou fanfictions or ask the community for suggestions.
One idea could be them competing in a cooking competition together, where they have to combine their different cooking styles. It would be full of tension and also the discovery of how well they work as a team.
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.