Exactly what is "speculative fiction" and how is it defined? Speculative fiction is to speculate on the final outcome of a situation, or to assume what will happen IF a certain event happens. Actually, a hint of fantasy is necessary to create speculative fiction. The need for this fantastical event is the pivotal point to make it speculative.
Every writing genre can be of a speculative fiction format, but normally it falls into the realms of science fiction, fantasy, horror and general fiction. The reason is simple: fantasy element.
Are you tired of playing semantics? How about some examples? When Jules Verne wrote his Twenty Thousand Leagues Under The Sea, the concept of a submarine at that time, 1869, was a very stripped, base version of today's vessels which still fall short of Verne's Nautilus. In fact, Verne's novel is still speculative fiction even in today's market since much of what he wrote of has yet to be proved or disproved; Atlantis.
In the 1971 movie, "Silent Running" starring Bruce Dern, a group of ships orbit the Earth. They are the last Garden of Eden chance for Earth, waiting to return and replenish the destroyed planet. Only one man cares about these unique environmental domes of flora and fauna. When the word is given to destroy them, he lashes out and attempts to save them with the assistance of his robot droids. Why would this movie be speculative? The movie's idea of environmental domes was a decade before the Biosphere's reality. Also, in the movie, the hero sends the domes off into space to become seed for Earth's second chance.
The 1995 movie, "Outbreak" took a speculative chance that a virus could destroy us in a very short length of time. Although not truly speculative since the right virus in the right environment could probably do that, the movie used it as the basis for its IF theory. Unfortunately, each year, the truth of that movie gets closer to fact than fiction: Ebola, SARS, AIDS.
Horror? Of course the horror genre uses speculative fiction. Using the "what if" scenario for speculative fiction: What happens when a pet dog goes bad? Read Stephen King's "Cujo" to find out. Read Michael Crichton's "Prey" to learn the secrets of nano-technology gone awry.
Blockbuster movies from books, Michael Crichton's "Timeline" and "Jurassic Park" use just a touch of whimsy to create a thriller. What happens when you mess with DNA from a prehistoric mosquito? In "Timeline" the pivotal point is obviously time-travel and Mr. Crichton grabs you by your bootstraps and sends you back and forth.
So exactly where can you find this marvelous thread to weave into your story? Read newspapers, scientific journals, even Wall Street. Somewhere, hidden within the myriad columns are articles that might just touch your imagination. It might be the most insignificant thing that you read, but it is what your mind can do with that concept that will create your story. An example: I was working on a story about time travel using a fabric. I was stumped when a friend handed me an article about a new material being used on bicycles. I haven't completed my story, but I have the article and my mind has raced with ideas of how to incorporate this new material into my story. Yes, it is lightweight and almost indestructible, perfect for a bicycle... or a time traveler.