I think resurrection could be a theme in the third trilogy, but not the return of Darth Vader. Just look at how important returning from the netherworld is in the series:
Episode I: Qui-Gon dies, his body remains as he did not fully become one with the Force. But his consciousness was saved. Later it is revealed that he can speak to others, but not appear in spectral form (until he is able to use the strong Force powers of Mortis)
Episode II: Qui-Gon's voice is heard, after Anakin loses his mother (and Anakin desperately wishes he could save her from dying, proclaiming that someday he will be powerful enough to do this)
The Clone Wars: Qui-Gon appears as a force ghost, but only on Mortis where the Force is incredibly strong. Anakin's mother returns as a ghost on Mortis (actually it is not her, it is a dark force manipulating Anakin, but it feeds on his angst that he could not save her from dying). Ahsoka dies but is brought back through the power of the Force, in large part due to Mortis but potentially in part due to Anakin.
Episode III: Palpatine teases Anakin with the knowledge of how to save those from death. Anakin foresees Padme dying, he sides with Palpatine but ultimately is tricked into joining the dark side. Yoda tells Obi-Wan that he has learned to speak with Qui-Gon, the two go into hiding while learning Qui-Gon's secrets.
Episode IV: Obi-Wan, filled with Qui-Gon's knowledge, becomes one with the Force and disappears. He immediately can speak with Luke.
Episode V: Obi-Wan appears before Luke.
Episode VI: Obi-Wan, once again, appears before luke after Yoda becomes one with the Force. Anakin sacrifices himself for Luke, killing Vader and in return Yoda and Obi-Wan make him one with the Force.
So we had voices in the PT, ghostly bodies in the OT, so the next step could be fully formed bodies in the ST. Potentially cheating death by becoming one with the Force, and being strong enough to manipulate the force to form a fully functioning body. Imagine if Luke dies in episode VII, and in episode IX he returns through the force.He would pretty much be the ultimate Jedi, and seemingly immortal. Able to come and go from the Force at will. Now THAT is something I'd pay to see.
And what if Palpatine died in Episode VI, but a part of his consciousness survived (sort of a Sauron thing, where he did die but a part of his soul remained) and Palpatine was trying to return by manipulating the Force? He wouldn't so much be a fully formed character as a dark force that is trying to return to the normal world.
I'd also really like to see a Force ghost Yoda and a Force ghost Anakin speaking with Luke or another Skywalker. Seeing Anakin and Luke sharing a scene would be a pretty powerful moment.