Tarkir block is the last block under the current system (three sets, fall-winter-spring), and it is going out with a bang. Not only is the first set wedge-themed, it's also featuring a time travel plot - something thought to be undoable in a Magic block after Time Spiral. Yet here we are, in the present of Tarkir, with five warring clans and extinct Dragons, knowing we won't stay there for long. We're getting Fate Reforged in the winter, and in the spring, well... that's "Louie."
Codenamed after Donald Duck's third nephew, we know three things about this set: It's large, it changes how drafting works, and it lacks the wedge color mechanical underpinning of the first set in this block. We don't even know what time period it's taking place in. But can we guess?
The near future, with alterations in the time stream by Sarkhan. Third sets that are large have to be that way for a good reason. The times this has occurred - in Rise of the Eldrazi and Avacyn Restored - something happens in the story that changes both the mechanics and the nature of the world in some way. Both of these, incidentally, have to do with something coming back that was once there - one has the Eldrazi arriving to annihilate, the other a freed Avacyn to save everyone. So the idea of Sarkhan changing the past so that Dragons can come back (the set name Dragons of Tarkir is a possibility) fits the established mold very well.
The far past. What more fitting irony would there be than to end at the beginning? Yes, a big, explosive set where the people of Tarkir are in the full swing of being terrorized by literal Dragonstorms - would make for an epic conclusion to this era of Magic's history.
The far future. As the clans seek the upper hand once and for all in their eternal war, they turn to researching draconic magic, and after generations become in harmony with the Dragons they slaughtered. Rather than a "battlecruiser Magic" style set with just lots of huge Dragons, imagine one with fantastical, pseudo-futuristic civilizations powered by draconic energies!