Yes, it can be a somewhat common theme. Many fans like to explore different relationships between the characters in fanfiction. Zoro and Robin are both popular characters in One Piece, and some fans might be intrigued by the idea of a romantic or affectionate moment between them, like a kiss. However, not all fans are in favor of this pairing, as there are various other preferred pairings among the fanbase.
Nami, on the other hand, would be more practical. She'd likely start planning for the future, like making sure they have enough resources on the ship for the baby. She'd also be thinking about how to balance being a mother with her role as a navigator in the Straw Hat crew. She might draw up detailed plans for the baby's living space on the ship and start thinking about teaching the child about navigation early on.
One possible fanfiction could be about Zoro and Luffy's unwavering loyalty to each other. Zoro is always there to protect Luffy and vice versa. They might face a powerful enemy that tests their bond, and through their combined strength and trust, they emerge victorious.
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.
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.
Their relationship would likely become more complex. Zoro, who is usually focused on his swords and his crew, would have to start thinking about someone else. Tashigi, as a marine, would also have to reevaluate her priorities. They might start to rely on each other more, sharing their fears and hopes for the future. For example, Zoro could offer Tashigi protection in a more personal way than before, and Tashigi could give Zoro advice on how to deal with the upcoming changes in his life.
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 common theme is the clash of combat styles. For example, when Zoro crosses over to a world like Naruto's, his swordsmanship clashes with ninjutsu. Another theme is adaptation. Zoro has to adapt to new environments, like if he's in a world with different magic systems or cultures.
Zoro is still alive and kicking in the One Piece manga. His character has overcome many challenges but hasn't met his demise. The manga keeps him going strong on his adventures.
Yes, there are likely fanfictions out there with such a plot. Fanfiction writers often explore different character relationships, and the relationship between Robin and Luffy is an interesting one to develop in non - canon stories.