Had things gone to the original plan, there would've been an appearance from a Force Ghost in The Force Awakens. Not just any old ghost either, but Anakin Skywalker, with Hayden Christensen reportedly lined up to reprise his role from the prequel trilogy. It was one of the many things that changed on the way to the film hitting cinemas, but it is something they could still revisit in Episode VIII and or IX. In fact, it would make even more sense now, because Luke Skywalker will be back in the fold properly, and presumably teaching Rey the ways of the Force, so an appearance from Anakin, Yoda, or Obi-Wan (or all three) is something that would fit with that. Whether Christensen should return is another matter entirely. I suppose if he doesn't have to do much but stand there, then he shouldn't be able to mess things up too much.