Did the Emperor anticipate his own death at the end of the Return of the Jedi? Yes? If he did, then perhaps his master plan was not finished yet. Perhaps there was a purpose behind his death that will be seen in future films. Because the Emperor pulled a bunch of strings in the Prequel Trilogy, so perhaps he continued pulling strings in the Original Trilogy as well. Maybe the Emperor had spies inside of the Rebel Alliance & was in fact still pulling strings in the OT just as much as he was pulling strings in the PT. Perhaps there are still mysteries that we are yet to discover until later films.