By the time Return of the Jedi rolls around, the Emperor's only advisor is Darth Vader, and his distrust in his organization is so complete that his only plan for succession is a desperate attempt to poach Luke Skywalker from the Rebel Alliance and get him to join his organization.
In A New Hope, the Galactic Senate is disbanded in favor of regional governors hand-selected by the Emperor. There is a constant through line we see starting with A New Hope and running through to the end of the Return of the Jedi of the Emperor consolidating more and more power into his own hands and that of his right-hand man, Darth Vader.
Perhaps the biggest mistake of the Galactic Empire made is its singular focus on the preservation of power for the Emperor and a few of his chosen lackeys.
Mistake #1: Building an organization around particular people, rather than institutions.
