According to
This Is Infamous, Luke will have followed in Yoda's footsteps and gone into hiding on Dagobah! And that this is where "he will be found and reunited with his old lightsaber by friends both old and new."
As for why Luke has gone into exile, according to
This Is Infamous,
"Luke begins to feel a disturbance in the Force… and starts to feel the lure of the Dark Side. If word about the Emperor’s resurgence is true, I believe his presence and desire to court young Skywalker once again is the root of this new evil. Because he knows just what a danger he would be if successfully convinced to switch sides, Luke goes into a self-imposed exile and disappears."
They also provide some details about Luke's increased abilities saying,
"From what I’ve been told, Luke hasn’t been missing for a full 30 years, only about 10, with his early time post-RETURN OF THE JEDI being spent rebuilding the Jedi Order. Yeah, Luke had some things to do, in order to help get things back on track after the fall of the Empire. In addition, he has grown so powerful with his command of the Force that he can literally move mountains or bring down ships with merely a glance."
While this is a tantalizing rumor (I've long thought that if Luke were to re-build the Jedi Academy he would do so on Dagobah, not Coruscant or Yavin IV), it does not line up with previous rumor that Luke would be found on the planet filmed at Skellig Michael (
rumored to be Mon Cala), which looks nothing like Dagobah. That's not to say this rumor couldn't be true and that rumor couldn't be false, but there is evidence that backs up the Skellig Micahel rumor, mainly that both
Mark Hamill and Daisey Ridley were spotted on location at Skellig Michael. Also, there's the whole "Emperor's resurgence" thing, anytime I hear that I get re-double skeptical-ness.