Warning! You are Entering Potential Spoiler Territory!
It sounds like Luke has been laying low and keeping things on the QT for the past twenty years or so since Return of the Jedi. That isn't to say he's just been sitting around in his robe and eating cereal, watching Clone Wars reruns all day. Far from it. It just might be that he's been putting together a grand plot to take down the Empire from within.
If the leaks are to be believed, Luke has been tucked away on Yavin and training potential Jedi's to be a part of an intricate scheme. Luke has a history of patience with cooking up these kinds of elaborate plots - think of that whole business with Jabba the Hutt in Jedi which required positioning Leia to be the big lizard's plaything.
Well it sounds like Luke has found someone else to infiltrate the bad guys this time around. Are you sure you want to know... ??
Luke and the 20 Year Plan
Ok! It could very well turn out that Luke has a son in Domhnall Gleeson - a son that he has trained and who has infiltrated the Empire and positioned himself among the Imperial Remnant. Not bad, Luke. In what is to be a turning point mid-way through the film, Gleeson will show his true colors, and the true color of his lightsaber, and join our main team of heroes.
It seems Luke has been training many people like this over the years - rebel soldiers of sorts who are positioned in various places waiting for their time to get called into action. Yes, Luke is working the long con here - a plot that is 20+ years in the making and is finally getting put into motion.
The Sith Retaliation
The rumor is that Andy Serkis' Sith lord doesn't take too kindly to these shenanigans and there's an explosive payback in store that brings Luke out from under the rock he's been hiding. Those X-Wing fighters you see in the trailer are supposedly from that scene, the cavalry that's zipping through Yavin to save the day.
The more I read these leaks, the more they start to sound pretty solid. Hopefully there will be a new trailer soon and we don't have to wait until May to see it attached to [The Avengers: Age Of Ultron](movie:293035). I'd love for some of these rumors to get backed up.
But I've read other leaks from other sources and it all matches up pretty well. And the main thing is, it all sounds pretty freaking cool!