One of my favourite concepts Rosewater explores in Making Magic is something called "design space".
In short, design space is what hasn't been done within a specific medium but that is a logical offshoot from what is currently available. It's the innovation that fills in the gaps. For example, if the first two Magic cards ever printed had been a red creature with 1 power and a red creature with 3 power, "red creature with 2 power" becomes an obvious card design.
How does this relate to fiction?
- Substantial portions of fiction, especially genre fiction, are effectively fictional adaptations of non-fictional subjects. Historical fiction adapts history. Most science fiction adapts physics, chemistry or engineering. Didactic instructional tales frequently adapt religion or philosophy.
- Which other subjects can be adapted? I've been doing this with diplomatic history. political science, industrial relations and economics in my upcoming novel, Burning Clouds. By digging deeper into these less-mined fields, I'm using design space.
- Related concepts can be substituted for each other. For example, houses can be made of wood instead of bricks. Positive: houses can be built faster. Negative: wildfires are suddenly terrifying. In fiction, this usually occurs more as "what if dragons breathed ice instead of fire?" or "what if people had flying cars instead of... non-flying cars?"
- I explore this concept in depth when discussing the replacement of fantasy-style magic with mysticism. To ape my above example, Positive: characters have greater reliance on technology, precluding Harry Potter-style wizard luddite-ism. Negative: battle scenes become less Tigana and more "let's scope out our enemies from a mile away and then lob artillery shells at them". I'm thankful I write such good diplomatic conference scenes.
- Fiction often borrows from other media. The possibilities here are endless. Borrowing from modernist literary theory? Why not also borrow from modernist architecture?
- I've done this in coining the term "literary hyperrealism". By combining Henry James-style realism, which I've been reading for years, with influences from hyperrealist painting, I've been working on a style (or is it a genre?) in which the action follows the characters so closely the reader can feel every breath, twitch or drop of sweat. See this post and this Quora answer for more.
Once you start thinking of established genres as hollow rather than solid, or as frames rather than as buildings, you can start filling in the gaps. Rather than describe someone else's world, you can squeeze into that spot within a genre, or between genres, to create your own.