Well, Palpatine didn't actually return from death in Legends. The 'Dark Empire' comics depict him transferring his spirit at the moment before his death into a clone body, but it's an imperfect method that causes each subsequent clone body to deteriorate more and more rapidly. He stages a whole return and reconquers the galaxy, but his focus becomes stealing a Skywalker body to solve the problem with his health, with the vague hope that someday he would conquer death. It actually fits very well with the prequels, and we know that Lucas at least read and enjoyed this comic (the various anecdotes seem to indicate that he had some interest in the comics and video games, but not the prose works), so there's speculation that it inspired some of the content in the prequels.
Darth Maul returns in the Clone Wars cartoon on Lucas's order, but that's another case of being powerful enough in the dark side to not have actually died.
The matter of Lucas's involvement in the Darth Plagueis novel is much more ambiguous, with some claims that he contributed "notes" in response to ideas, but the author never actually had any direct communication with him. I made a tumblr post going into a lot of detail on the subject of that immortality, but what we actually see Plagueis do in his novel is more about enhancing the physical body through the dark side. Even the cases where he brings someone back to life is pretty much a medical resuscitation. So, like with Palpatine in the comics, we see a fight against death, but not the ability to come back once death has won. That's the difference between the Jedi and the Sith. One accepts death, and in that way finds the way back from it. The other fights against it, and in doing so creates a situation where it will eventually win.
Now, Legends did have dark side Force-ghosts, and they were usually depicted as being anchored to some specific point and being actual haunting-style ghosts. That does create a difference from the Jedi way of ghosting, but as long as we've trashed the Legends material, I'd rather go with a depiction that adheres to the thematic message of the movies and doesn't try to skate by on a technicality.
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Darth Maul returns in the Clone Wars cartoon on Lucas's order, but that's another case of being powerful enough in the dark side to not have actually died.
The matter of Lucas's involvement in the Darth Plagueis novel is much more ambiguous, with some claims that he contributed "notes" in response to ideas, but the author never actually had any direct communication with him. I made a tumblr post going into a lot of detail on the subject of that immortality, but what we actually see Plagueis do in his novel is more about enhancing the physical body through the dark side. Even the cases where he brings someone back to life is pretty much a medical resuscitation. So, like with Palpatine in the comics, we see a fight against death, but not the ability to come back once death has won. That's the difference between the Jedi and the Sith. One accepts death, and in that way finds the way back from it. The other fights against it, and in doing so creates a situation where it will eventually win.
Now, Legends did have dark side Force-ghosts, and they were usually depicted as being anchored to some specific point and being actual haunting-style ghosts. That does create a difference from the Jedi way of ghosting, but as long as we've trashed the Legends material, I'd rather go with a depiction that adheres to the thematic message of the movies and doesn't try to skate by on a technicality.