Perhaps in this fanfiction, their pregnancies are the result of some strange magic or phenomenon on a mysterious island. Zoro could be initially in denial about his situation, but as time passes, he starts to feel a sense of responsibility. Robin, being the more calm and collected one, would start preparing for the arrival of the baby. She might share her experiences and knowledge with the other female characters in the crew, like Nami. The whole crew would then have to face new enemies who are after the unborn children for some unknown and dangerous reasons.
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.
A plot for this fanfiction could be that Robin and Zoro keep the pregnancy a secret from the crew at first. But as time goes on, it becomes harder to hide. Meanwhile, they are also facing a new island with strange customs and threats that are related to Robin's pregnancy in some way. For example, the island has a law that pregnant women must perform a certain ritual, and if they don't, something bad will happen to the baby.
The plot could revolve around a curse that was placed on them during one of their adventures. This curse made them pregnant. Zoro and Tashigi would then have to team up to find a way to break the curse. Along the way, they might discover secrets about each other that they never knew before. For example, Zoro might learn more about Tashigi's past and her dreams outside of being a marine. And Tashigi could understand Zoro's loyalty to his crew on a deeper level. Their journey to reverse the pregnancy could also lead them to new and unknown parts of the One Piece world.
Expect a lot of character exploration. Since Robin has a complex past and Zoro has his own goals and dreams, the fanfiction could delve into how they influence each other. For example, Zoro's pursuit of becoming the world's greatest swordsman might inspire Robin to pursue her own academic or personal goals more vigorously.
One idea could be that during a dangerous adventure, Zoro and Robin get separated from the crew. They have to rely on each other's skills. Zoro's combat abilities and Robin's knowledge of ancient texts help them navigate through an ancient, forgotten island filled with mysteries and traps.
In Robin x Zoro fanfiction, their relationship can be really interesting. Robin is the intellectual and mysterious one, while Zoro is the strong and determined swordsman. Their interactions might start from a simple comradeship on the Straw Hat crew. Maybe in some fanfictions, Robin's knowledge could help Zoro in his swordsmanship journey, like providing him with historical information about some special swords or fighting techniques from the past.
Their different personalities make them an interesting pair. Zoro is the tough, straightforward swordsman, while Robin is the intelligent and mysterious archaeologist. In fanfiction, this contrast can lead to some great character - driven stories. For example, Zoro's simple view of right and wrong can clash or complement Robin's more complex understanding of the world's history and secrets.
One great story could be about Zoro and Robin teaming up for an archaeological discovery. They might find a long - lost poneglyph that holds the key to an ancient civilization's power. Zoro would use his strength to protect Robin as she deciphers the inscriptions, and their bond could grow stronger through shared adventures and mutual respect.
In this fanfiction, the pregnancy could be a result of a deep emotional bond that developed between Zoro and Robin over time. The crew might be initially shocked but then very supportive. The baby could become a symbol of a new beginning for the Straw Hats. There could be challenges like protecting the baby from other pirates or the World Government who might see the child as a threat due to its parents' reputations. Also, the baby could bring out a softer side of Zoro and Robin, and they could learn a lot about themselves as they become parents.
One possible storyline could be that they accidentally got a magic spell cast on them which made them pregnant. In this fanfiction, they would have to figure out how to deal with this unexpected situation while still being part of the Straw Hat crew. Maybe they'd struggle to keep up with their pirate duties at first, but then find ways to adapt and still contribute to the crew's adventures.
One possible plot could be that Robin realizes her feelings for Luffy during a particularly dangerous adventure. Maybe they get separated from the crew, and in the time they are alone, she sees a different side of Luffy and her feelings start to change.