All other genres of literature, except for science fiction, must follow certain rules: mainstream fiction must conform to reality as we know it, even if that reality is presented from the point of view of a madman (Pale Fire). Romance fiction must feature, among other things, a romance (Wuthering Heights); and mystery fiction must have a mystery to drive the plot (Farewell, My Lovely). With science fiction, things are not so cut-and-dried. In fact, there is only one real rule that sf writers ignore at their peril.
The most important rule for writing science fiction is that a sci-fi story must be self-consistent. You can write a story that takes place in the present, past, or future. You can write a mystery or a romance or both, within that sci-fi world. You can write an alternate history--what would have happened if Germany had won WWII? You can write an adventure that takes place on another planet or in another galaxy. In good science fiction, almost anything goes, as long as it is explained within the parameters of science as we currently understand it, or the science of another universe or dimension. As long as this rule is observed, sci fi writers have virtually unlimited freedom in what to write about. For example:
It may be science fiction, but it can take the form of a mystery, a romance, or a spy story. For example, in my science fiction series The Pandora's Trilogy, the driving motivation is a romantic triangle involving two men and a woman. The most common form of marriage in the Pandora's world (our present world about 100 years from now, after a genetic disaster destroyed all technology) is between two or more men and a woman (for the good reason that, due to a genetically transmitted disease women are vanishingly scarce in this world).
You can explore any topics that interest you, as long as they fit the basic scientific premise of your work. For example, my future world has several empathic animals that can communicate more or less telepathically. I knew from previous research that some elephant scientists speak of elephants "as if they could read thoughts;" and there is a great deal of evidence that some animals including household pets possess seemingly supernormal abilities--finding their way home from a great distance, or sensing the approach of an earthquake or other disaster. I simply posited that in the world of my books these creatures had developed their latent abilities as a result of the genetic disaster.
You can express political or sociological points in a relatable way. Such famous dystopian novels as 1984, Brave New World, and Stand on Zanzibar seem to have eerily predicted the last part of the twentieth century. The authors had closely observed the society around them and extrapolated trends to create their science fictional worlds. One of the things that most concerns me in our present world is the massive, accelerating extinction of animal species.
At one point in the third novel in the series, Pandora's Promise, Zach, one of the three viewpoint characters, proposes enlisting the empathic elephants in a military scheme to restore civilization. Another character, Jonna, replies that the elephants would not cooperate, because "I do understand, and so do the elephants, that human civilization has meant nothing but misery for the animals of this earth.” “It is true that humans caused the Change,” Zach said. “But perhaps it has benefited the elephants, enabling them to develop advanced intelligence and to enter into other creatures’ minds.” Again Jonna was silent, and when she answered she sounded amused. “Why do you believe that the elephants did not possess those abilities all along, even before the Change?”
Because it is FICTION, you can explore topics that might get lost in a nonfiction piece. Instead of writing an article about the terrible suffering of "trained" elephants in circuses and zoos, I can simply SHOW that suffering from the point of view of the elephants. (See [Rushing River]'s Story, from Pandora's Promise.) Thus a point that might be skipped or glossed over by a reader of nonfiction will be made viscerally, once the reader has become interested in the elephants as characters in a novel.
