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.
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.
Well, 'chopper fanfic' is basically fan - made stories that revolve around the concept of 'chopper'. If 'chopper' is a vehicle, like a motorcycle (which is often called a chopper), then the fanfic could be about cool rides, customizations, or races involving these choppers. It all depends on what the 'chopper' in question represents in the context of the fanfic.
Hard to say. It could be associated with a minor character whose background wasn't fully explored. Or perhaps it was just a random prop without a specific owner.
Yes, Chopper might like to read stories. Maybe he likes adventure stories as he is quite an adventurous character himself. Stories about exploring new islands or facing big challenges could be his favorite.
I'm not sure specifically which 'chopper sad story' you mean. There could be many stories involving something named 'chopper' that are sad. It could be about a pet chopper that got lost or a vehicle chopper that was damaged beyond repair. Without more details, it's hard to tell the exact story.