Generally doomsday theories are governed by religion, each religion will have its own apocalypse scenario. (Christianity has basically the book of revelation, Vikings had Ragnorak) But with the expansion of the entertainment media many new doomsday theories have arisen in popular culture (zombie apocaylpse, meteorites) You could also mention the "Great Dissapointments" (When people expected the world to end on a particular day, and it didn't) There are also doomsday theories that are scientificly possible, but highly improbable and generally unknown about (gamma burst radiation I think is one)
Edit: In your speech you could talk about how doomsday scenarios have changed through time, from religious, based on faith, which held lots of fear and were widely believed and feared in a society, to popular culture, based on pseudo-science (2012) which is believed by some but not others in our culture, to scientific, which are understood, believed and lagely ignored. Just a thought I had, but you probably have a more suitable framework set up.