I also see that they made intentional parallels between Yoda and the Emperor, as far as being the masters of their respective apprentices. Some of it is more apparent in Jedi, but it certainly starts in Empire. Frail, wrinkled, very old figure who walks with the aid of a stick. Smaller than their pupil. The use of both the words “master” and “friend” to describe their relationship (friend might not seem like a giveaway, but Lucas frequently links the last line of one scene with the first line of the next scene – “I have a promise to keep… to an old friend” / “Rise, my friend”)

It seems like as they were developing a master/pupil dynamic for Yoda and Luke in Empire, they were doing the same for the Emperor and Vader. The Emperor just didn’t get much screentime until Jedi.