Fantasy fiction is created by imagining impossible things, a technique at the heart of all the genres that can be loosely grouped as fantasy. If this is a field you enjoy writing or reading then you will delight in this study that traces the history and development of fantasy and looks at how the conventions of the genre have evolved.
Back in the 18th century, we find fiction displaying a spectral supernatural focus with its familiar props – the haunted castle, the night-time graveyard. Much more recently we find more sophisticated horror fiction in which human existence is seen as a commodity traded by blind and uncaring forces. At the same time, the number of sub-genres has proliferated to include such things as urban fantasy, magical realism, paranormal romance, and even a flavour of dark horror which has been termed splatterpunk.
If you are going to write fantasy it seems that your subject matter is going to be whatever makes contemporary society uncomfortable.