There are indeed. A lot of fanfiction writers are attracted to the dynamic between Rock and Revy. Some might take their relationship in a romantic direction, imagining how they deal with their feelings in the middle of all the chaos in their world. You can search for them using specific tags on relevant platforms.
One common theme is the exploration of their opposite personalities. Revy is wild and impulsive while Rock is more calm and rational, and fanfics often play with how these differences interact.
Archive of Our Own is a great place to start. It has a large collection of fanfictions, and you can easily search for Rock x Revy ones using the search bar.
Another factor is the world they exist in. The dangerous and gritty setting of their story provides a great backdrop for fanfic writers. They can create all sorts of scenarios where Rock and Revy have to rely on each other. This adds an element of adventure and drama to the fanfics, making them very appealing to readers.
One great Revy x Rock fanfiction is 'Shadows of the Lagoon'. It delves deep into their complex relationship. The story shows how Revy's tough exterior starts to crack around Rock. Another interesting one is 'The Unspoken Bond'. It focuses on the unsaid feelings between them during their dangerous adventures in the world of Black Lagoon.
Another theme is the idea of redemption and growth. Revy has a dark past, and in some fanfictions, Rock becomes a catalyst for her to change and find a new path. It's about how their relationship can be a force for positive change for both of them.
A common theme is the exploration of their complex relationship. Revy is tough and Rock is more of an ordinary guy in a sense, so the fics often play with how they interact and grow closer. Another theme could be adventure, as they are in the dangerous world of 'Black Lagoon' and fanfics might send them on new and exciting escapades together.
One great Rock x Revy fanfic is 'The Unlikely Bond'. It delves deep into how their contrasting personalities start to find common ground. It has some really interesting character development moments that show Rock's calmness slowly influencing Revy's more impulsive nature, and vice versa.
I'm not sure specifically about 'Rock x Revy pregnant' fanfiction. You could try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net. Maybe look for tags related to the characters and the concept of pregnancy to find relevant stories.
Their relationship can be developed through shared danger. For example, if they are in a shoot - out together, Rock could save Revy in a non - traditional way, like using his knowledge of the environment to outwit the enemy instead of using guns. This would make Revy see Rock in a new light.