Predicting the End of the World
The Hunger Games is set in a very unique post-apocalyptic setting, having no distinct causation as to what happened to cause the end of the world, only the condition that it is "the end of the world" per say.
In terms of whether I personally believe in the "ending" of the world, any ending of the planet's current habitation would require significant change of some sort - something to trigger an event happening. Many of the suggested examples (nuclear or chemical warfare) require some sort of man-made motivation for entering such a destructive situation.
I am fascinated (narrative-wise) by the idea of alien invasion - the idea of encountering a being/species so unlike our own is a very intriguing idea for a Cinema major, rooted by the childlike fantasies of meeting an alien! However, scientists haven't been able to discover (yet) any intelligent life in the outer reaches of space, therefore this notion, while exciting, can be ruled out.
The most probable way the Earth would end, in terms of humanity, would bed climate changes. History has shown that climate has had major impact on many creatures of the Earth in terms of extinction and endangerment. The dinosaurs are the most famous example of a species of animal wiped from the Earth; a species that survived for thousands of years before being entirely obliterated from the planet.
There's also the threat of humans destroying themselves environmentally. Countries like China and even regions of the US have so much pollution, they have to wear masks to save themselves from the smog. It is most possible that humanity takes this pollution (and possibly global warming?) to the extremes, making the planet inhabitable for us to live on.