My Poli Sci professor (and other authors like Kaldor, blah, blah, etc.) says the next world wide threat will be resource scarcity caused from over-exploitation and global warming and this will cause the subsequent disestablishment of states unable to control groups competing for resources. All of this leads to good stuff like an influx of too much migration to the western states, and an increase in generalized terrorism and paramilitary groups perpetuating violence and crime for profit (not just islamic terrorism). Global warming could also be a threat to the "western world" by itself as well.
That, or dinosaurs.
edit: But based on the stuff I've read, I wouldn't call Islamic terrorism the current "bogeyman". Some of it all actually requires serious action, not for us to put our heads in the sand.