H.G. Wells is considered the godfather of science fiction, and for good reason. With this title he had established a single rule to stop the genre of science fiction from being dragged into the realm of fantasy.
This rule was,"A science fiction story should contain only a single extraordinary assumption." This rule allows for the scientific advancement of stories to be limited within reason and allow the flow of what if.
Aliens come to attack the earth: This topic has been explored over and over, since the beginning of the concept that their are aliens, the concept of dangerous ones followed immediately. this idea is key in works like "War of the Worlds", "Alien", or "The Three Body Problem".
2. Wormholes and Black holes: The interesting thing about these marvels of science is the fact that truly no one knows what they are. The idea that they are a pathway that bend space and/or time is something plausible, which is what makes this trope so fun. This idea is explored in pieces like "Interstellar", "Wrinkle in Time", or "Vorkosigan Saga".
3. Killer robots: Based around the idea that the machines we create will one day turn on us in an attempt to rule their world. This form of story line is best related to the stories of Issac Asimov, a renowned robotic sci-fi writer who developed the three rules of robotics that are spread widely amongst his works and other sci-fi writers.
4. Android Revolutions: In this scenario there is the set idea that robots and AI will turn fully sentient, and with this new found consciousness and humanity, their is a want for freedom and equal rights amongst humans. This idea of a slow developing humanity and a want for freedom is developed throughout "2XYX", "Detroit:Become Human", and "I, Robot".
5. The creation of life: Its a common enough idea, what would happen if humanity was able to create life. This topic is what leads to one of the most well known stories of science fiction, Frankenstein. The theme of a scientist's creation turning on them in the hopes of freedom and acceptance is well explored and loved by many and well explored in "2XYX".