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Is there a limit on suitable subjects which could be used for commercially marketable fiction, or can it be about anything?
For instance, would it be possible to write a story about a man who lives alone having a simple meal of ham and mashed potatoes? Or about someone just sitting and reading the morning newspaper?
Theoretically fiction can be about anything the writer wants.
A story (possibly not a novel) about a man reading a newspaper might be very popular with a certain sort of critic.
On the other hand it might not. But if it's what you want to write....
You mean like one about some jerk eating a madeleine?