The series will mine material from the 19 years that follow Episode III: Revenge of the Sith and precede the Episode IV: A New Hope. In other words, using the life of the Skywalker twins as a measure, the series will be active in the story years that come after their separation as newborns and before their (unwitting) reunion on the prison level of the Death Star.
Star Wars Rebels just gets back to brass tacks, pushing much of the show's arcs to the side to focus on one: Sabine, the Mandalorians, and the vague but powerful weapon she helped create.
As long as those endings are satisfying, as long as they honor all of the wonderful characters, Rebels' four-year journey will have been a successful one.
