More of a misunderstanding than a misconception, but I used to think “There is another” referred to Darth Vader, because he ultimately is the one who defeats the emperor and I thought Yoda had the foresight to see this. Mind you I was very young at the time and the only prequel that had yet been released was TPM. Looking back, my misunderstanding sounds like something GL would ret-con into being “his original intention” nowadays.
I think this is just evidence of you young Darth Lucas being more insightful than middle-aged George Lucas.
Using this interpretation you can short cut the originally intended sister plot to have ROTJ as the ‘final’ episode without the need to make Leia Luke’s sister.