One of the 4 fun strategies for writing explained on the Ways to Inspire Stories page is using imaginary worlds. Imaginary worlds are any nonexistent place that someone writes about. Some authors even have a picture of the world surrounding their characters in their books. These are just a few imaginary worlds (or realities) that could be used in stories.
Tarmax~ A planet far from any other inhabitable planet. It can support life, but only certain species. Only birds are able to survive there, so there are many unique species of birds. (Dodos and passenger pigeons also exist on this world, because they didn't become extinct there.)
Intra-earth~ A version of futuristic Earth where explosions destroy (most of) the top layer of the planet, (and all of the life on it is either wiped our or goes into hiding for a couple years until the trouble passes). When the creatures finally come out there are very few of them, and everything around them is destroyed. The plant life is diminished to a few tough patches of brush, or some trees and grasses that happened to have been in places that weren't completely destroyed.