One common theme is friendship. Luffy and Chopper have a very special bond in the original series, and fanfics often explore and expand on that. They might write about how they support each other during tough times on their adventures.
A great one is 'The Duo: Luffy and Chopper'. This fanfiction focuses on their teamwork. It portrays how Luffy's fearless nature and Chopper's cute yet determined personality blend well. They often get into comical situations like when they try to steal food from a big - time pirate's ship just because Luffy was hungry and Chopper didn't want his captain to starve.
You can try fanfiction websites like Archive of Our Own (AO3). There are often a lot of fanfics about Luffy and Chopper there. Just use the search function and type in 'Luffy x Chopper' to find relevant works.
Often, it emphasizes their friendship. Nami's feisty and smart nature combined with Chopper's innocent and helpful self makes for interesting interactions. For example, in some fanfics, Nami uses Chopper to get out of tricky situations with other crew members by pretending he needs her help immediately.
Growth is a prevalent theme. Chopper, as a character, is constantly evolving, and Robin plays a role in that in these fanfics. They also often have themes of discovery. Whether it's exploring a new island or uncovering a mystery, they do it together. Their relationship is at the center of these stories, and the exploration of their different personalities is also a common aspect. For instance, Chopper's innocent and curious nature against Robin's more reserved and intellectual one makes for interesting interactions.
Friendship is a very common theme. Since Robin and Chopper have different personalities, their friendship is often explored. For example, Robin's wisdom can guide Chopper, and Chopper's pure heart can bring out Robin's softer side.
One common trope is the 'fish out of water' situation. The OC, being new to Chopper's world, might struggle to adapt to the strange and often dangerous pirate life. For example, they could be shocked by the battles and the rough nature of the crew at first. Another trope is the 'healing relationship'. Chopper, being a doctor, could heal the OC not just physically but also emotionally, and in turn, the OC could help Chopper grow in confidence and understanding of human relationships. There's also the 'adventure buddies' trope where they go on various adventures together, facing challenges side by side and relying on each other's unique skills.
There are also fanfictions that focus on the emotional support they can offer each other. Chopper, being the doctor, can take care of Robin when she's injured, and Robin, with her knowledge and wisdom, can be a source of comfort for Chopper when he's doubting himself. This mutual support often forms the basis of a heart - warming relationship in the fanfictions.