One line that seems to carry a lot of importance in ESB but doesn't really feature elsewhere is Yoda's lesson of "Do or do not, there is no try".
It would be good if we saw the failures of Ben, Anakin, the Jedi, the Republic etc coming from them trying and failing and the successes of Palpatine, Vader, etc coming from them doing without trying.
Ben tries to teach Anakin, Anakin tries to be a Jedi, the Jedi try to defend a Republic trying to maintain justice and control in the galaxy but they lack faith in their abilities that is why they fail.
Once Palpatine is on top he becomes complacent (his overconfidence is his weakness) and Vader returns to a position of doubting if he could ever overthrow his master ( "You don't know the power of the darkside I must obey my Master") they both stop doing and start trying to overcome the Rebellion that is why they fail and the Alliance ultimately win.
Even Luke almost fails because he tries to turn his father back to the lightside instead of just doing it.