One interesting storyline could be Chopper discovers a new type of medicine on an island and Robin helps him research it. Their different skills, Chopper's medical knowledge and Robin's ability to read Poneglyphs, combine to unlock the secrets of this new medicine which could potentially save many lives in the One Piece world.
One way to develop their relationship further is through cultural exchange. Since Chopper is from Drum Island which has its own unique culture and Robin has traveled to many different places, they could exchange knowledge about different cultures. Robin could teach Chopper about the ancient cultures she has studied through Poneglyphs, and Chopper could share about the traditions on Drum Island. This would not only expand their knowledge but also bring them closer together as they learn more about each other's backgrounds.
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.
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.
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.
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.
You can try some popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of various fanfictions, and you might be able to find some great Chopper x Robin ones by using the search function.
One popular 'Robin x Chopper fanfiction' is 'Adventures of Robin and Chopper'. It has a really cute storyline where Robin helps Chopper explore new islands and they form a special bond. The author does a great job of portraying their characters in a way that is both true to the original anime but also gives them new depth in their relationship.
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.
Many 'One Piece Robin x OC fanfiction' works explore the theme of acceptance. The OC, being an outsider in the One Piece world, has to be accepted by the characters, especially Robin. And Robin, with her own past, also has to accept the presence of this new character in her life. There's also the theme of growth, both in terms of power and character, for both Robin and the OC as they interact and go through various experiences together.