Warning: spoilers for Star Wars: The Force Awakens ahead.
In a word: Immortality.
This is a set up Star Wars made all the way back in 1977 when Alec Guiness first uttered the words: "Strike me down and I will become more powerful than you can possibly imagine." And we have yet to see the payoff.
We've seen glimpses. Moments after he's been reduced to a pile of smoldering robes, Obi Wan is back, telling Luke, "Run Luke! Run" He helps Luke blow up the Death Star, and in Empire Strikes Back, he appears to Luke on Hoth, and later, Dagobah. In fact, Obi Wan is a central character in films 1-6 -- The Force Awakens being the only film where he doesn't appear on screen. (Although we do hear him in Rey's so-called "Force Back" vision.)
When the consciousness of a Jedi Master survives death, that Jedi Master can come back to guide the living. This is a big deal. It's a tremendous advantage for the Rebel Alliance in the original trilogy, and it should be a continuing advantage for the Resistance in the new films.
In fact, it's such a big advantage that one has to wonder how the Empire morphed into this First Order beeswax in the first place. I mean, come on. What, Obi Wan can tell Luke to "Run Luke, run!" but he can't help them to prevent the whole StarKiller Base thingy? In fact, there's only one scenario where this seemingly planet-sized plot hole can be filled: The bad guys have the same advantage as the good guys.
"Did you ever hear the tragedy of Darth Plagueis the Wise?"
The prequel trilogy may have been disappointing overall, but there were moments that kept fans salivating for more. One such moment was Chancellor Palpatine's play for Anakin's soul, in Revenge of the Sith. Palpatine tells Anakin that Darth Plagueis the Wise could "cheat death." He goes on to explain that Plagueis could also use the Force to "manipulate life." Most of us probably figured he was lying; after all, it was what Anakin wanted to hear in order to save Padme. But then again, what if he wasn't lying? What if all of it were true?
Hold onto your hat. This is where it gets weird.
Anakin was conceived by Plagueis's manipulation of the Force. He's got Dark Side DNA coursing through his bad ol' self.
So much for the "Chosen One." If Anakin truly was created by a Sith Lord, then he was destined to fall to the Dark Side. And it gives new meaning to all that talk about Luke's destiny too. But then, if Anakin is the product of Plagueis' midichlorian tampering, why haven't the good guys gotten in on that? I mean, we've got Force Ghosts who should know about this stuff, right? Well, maybe they do. Maybe Yoda and Obi Wan are able to share what they've discovered with Luke between the events of Return of the Jedi and The Force Awakens. Maybe this is the big reveal of Episode VIII -- Rey was conceived by Luke's manipulation of the Force.
If you think about it, it makes sense. Star Wars has to address the issue of Force Ghosts, going forward. You can't populate your universe with messengers from the beyond and then sweep all that under the rug. It's too big a plot hole. So yes, we can expect (going forward) to hear from Yoda, Obi Wan and Anakin. And I expect we'll also hear from Palpatine, Vader -- and Plagueis. (I think he's the big bad lurking in the shadows for this trilogy. Perhaps he's Snoke, or communicating with Snoke from beyond the grave.)
If we assume that the writers for the new trilogy will address the issue of Force Ghosts, then we'll need to ask what role they will play. What would these masters have to say to guide the next generation? And the answer to that is: What plot seeds have been planted, but haven't yet come to fruition? As it happens, there are two big ones.
From beyond the grave, we can assume that Yoda and Obi Wan are privy to all sorts of cool knowledge. Such as, for instance, Anakin's true parentage, and the nature of that pesky prophecy. If the Skywalkers have Dark Side DNA coursing through their veins, then Luke's resistance of the Dark Side becomes that much more significant. (Leia was never trained, perhaps we now know why.)
As Force Ghosts, Obi Wan and Yoda know Plagueis set Anakin on the board, it's only natural for them to help Luke create their own champion, an incorruptible Force-conceived Jedi, born for the purpose of taking down the bad guys once and for all. We've seen the kind of damage a Dark Side Force-conceived Anakin wrecked on the galaxy -- now let's see the kind of good a Light Side Force-conceived Rey could have on the future.