A "black swan" is an event that is highly unlikely and therefore often totally unexpected but one which is still completely possible. It is also often a complete game changer as far as international politics, etc. goes.
An example often used is a global pandemic, something as lethal as the 1918-1920 flu epidemic that affected about a quarter of the people in the world and killed about fifteen percent of those who got sick (50-100 million). The sudden death of a world leader (possibly by assassination, but plenty of people just have heart attacks, too) is another possibility.
So, just for fun...what do you consider the black swans that could lead, directly or indirectly, to scenarios.
Here's one for starters...Vesuvius. What if it goes of in 2016 or 2017, but with relatively little warning, or during a period when the Italian government is in turmoil for some other reason and does not act as quickly as it needs to? So we end up with a major eruption (4 to 5 VEI, the 79 eruption was a 5) before most people can be evacuated. The results...the city of Naples is effectively destroyed, tens of thousands are killed (maybe more since 600,000 live in what is called "the red zone"), and the damages are in the billions of euros (and perhaps tens or even hundreds of billions--what is a city like Naples worth?). How would this affect Italy? And how would it affect the rest of the E.U.? What does the E.U. look like in 2020 if something like this happens?