Yes, there are quite a few. You can start by looking on fanfic websites like Archive of Our Own. Some fanfics might focus on their adventures together on the Thousand Sunny, exploring how their different personalities interact during various situations.
Archive of Our Own (AO3) is a great place to start. It has a large collection of fanfics from various fandoms, including One Piece. You can simply search for 'Franky x Robin' in the search bar and it'll show you relevant stories.
Well, perhaps in the fanfiction, Franky and Robin's relationship deepens during an adventure on a new island. They find out about the pregnancy there. Franky might be a bit clumsy at first, like trying to build a too - big crib that takes up half of the living quarters. Robin would be the one to keep things in order. The crew would all pitch in to protect Robin during her pregnancy, with Sanji cooking special nutritious meals for her.
Their relationship could grow stronger as they face the challenges of pregnancy together. Franky would become more protective, using his mechanical skills to ensure Robin's comfort. For example, he could build a special chair for her. Robin would rely on Franky more, sharing her fears and hopes for their future family.
One possible plot could be that they are on a new island during their adventures in One Piece. Since they are pregnant, the crew has to be extra cautious. Zoro might struggle with the idea of being a father as he has always been focused on his swordsmanship. He could start training in a new way to protect his future family. Robin, on the other hand, would use her knowledge to ensure a safe pregnancy. She might research about any potential threats to the babies from the various historical texts she has access to. The crew would also rally around them, with Sanji cooking special nutritious meals for the expecting parents.
One possible idea for a fanfiction could be that Robin discovers she's pregnant during a time of relative peace on the Straw Hat ship. She might initially be worried about how the crew will react, especially considering their adventurous lifestyle. But as time passes, the crew shows overwhelming support and they start making preparations for the baby on board, like creating a safe nursery area. This could also lead to some comical moments as the usually rowdy crew tries to be more gentle and quiet around Robin.
One interesting Franky fan fiction could be about his adventures before joining the Straw Hat crew. Maybe it details his time building those amazing inventions in Water 7. Another could be a story where Franky uses his skills to upgrade the Thousand Sunny in a completely unexpected way during a tough battle. And there might be a fan fiction that focuses on Franky's relationship with other crew members, like his bromance with Usopp and how they come up with new gadgets together.
I like the storylines where Robin forms deep friendships in the fanfictions. In one, she befriends a group of scholars on an island. They exchange knowledge and she helps them protect their research from pirates. It's a great display of her character values. There are also storylines that explore her relationship with the other Straw Hats in new ways, like how she teaches Usopp about ancient plants.
One popular storyline could be about a female character getting pregnant during an adventure. For example, Nami might find out she's pregnant while on the ship, and the crew has to adjust to this new situation while still facing pirates and other challenges. It could add an interesting dynamic to their relationships and how they operate as a crew.
Robin and Luffy have some great interactions in fanfiction. Luffy's silly antics can often make Robin laugh, which is a rare sight given her usually calm demeanor. For example, in some fanfics, Luffy might accidentally stumble upon Robin's reading corner and start asking her all sorts of curious questions about the books, leading to an endearing exchange between them.