''SomeNorCalGuy'' on Reddit wrote something that seems very likely to me:
''Well I'm just going to pretend that instead of pulling this title out of a hat, JJ Abrams is trying to tell us something about what to expect here.
In the prequels, ~30, 40 years before Ep. IV, we saw maybe a dozen or so true Jedi which were representative of hundreds, perhaps thousands of Jedi and others perceptive of the ways of the force. In the original trilogy it's widely believed that Vader is all that remains of their ancient ways (if you're counting though, there's still only 6 force perceptive individuals in the movies: Vader, Palpatine, Yoda, Obi-Wan, Luke, and Leia).
So I think to make up for lost time and the silencing of thousands of voices we're going to going from two Jedi (Luke and Leia) to a few Jedi (Luke and Leia's kids) and then Luke (who is the one who is supposed to bring balance to the force) will be using super-enhanced force powers and sensitivity to those with force perception to bring about a reconstituted Jedi Order of dozens, maybe hundreds of people all across the galaxy.
BUT...
There's still the Sith out there. Always two there are - master and an apprentice. Vader had an apprentice out there. And Luke knows it. And try as he might, he can't find him. But he's out there, growing older every day and he needs an apprentice.
The force is rising. For both the Jedi and the Sith, the force is becoming more concentrated; more powerful. More benevolent and more destructive. This is that first story. The rise of the power of the force, the reconstitution of the Jedi and, perhaps most importantly, the rise of the Sith.''
Even though I do not completely agree with the last part about the Sith, I think the rest sounds reasonable. Sith could also feature due to ancient Holocrons with teachings, or Nightsisters could be involved.