This post,'The fallacy of difference, in science and art', puts a proper label on something I've been mulling over for a long time.
Fallacy of Difference.
We focus on what's unique, or defining, and forget commonality.
A character whose defining attribute for my story is her supportive role for the protagonist is still a real person. I ask myself, not "what would a supporting character do?" but rather, "What would Sara do?", because she is real to me. But I see where I have failed to create this reality in other characters.
It holds for settings, for ideas, for plot. Nothing should be defined by its uniqueness. That is one dimensional. Strive instead for facets that reflect points of view. Create a virtual reality or a hologram, not a flat, static image.
Three dimensional writing - no, has to be four. Time, and the changes time brings are essential in life and in art.