There are many interesting possibilities. I have scenarios that cause partial reduction in human population as well as those that would have complete or near complete extinction. These have varying degrees of scientific background, but all have been either implied by current science, been thought possible, or are creations of my own, based on readings and research. A friend stated some time ago that human kind was its own worst enemy. He was talking about the ecological impact the human community is having on its own survivability. I was moved to disagree, and offered the view point that nature still has more ways of doing in humanity than humanity does. That discussion prompted some research and so I have made a study of "Extinction Level Events" The items in my collection of possible events can provide a good vehicle for the writer as well as the researcher.
Currently we have 15 scenarios in the collection with a few more being looked at. As I have said some are minor level involving a decrease of maybe one or two percent in the human population, whereas the others, some of them, are quite comprehensive and, interestingly, quite possible. A select few of those are the kind that would at once be quite swift and provide little or no warning,
If you are interested in discussing these scenarios, I am interested in finding a way to make use of them through others.