I really think it's more interesting to keep the reveal of Palpatine for ROTJ. I think the saga, being approached as the story of the rise and fall of the Empire, can afford to strip away the layers slowly. By the end of the PT, we can be in no doubt as to the Sith's involvement with the Empire, but the fact that Palpatine himself is a Sith Master need not be known until ROTJ. I like the idea of Vader disappearing while still Ben's apprentice and then returning as a Sith Lord, leaving us still wondering about the levels to which the corruption reaches and how much it will take to bring it down.