One of art legend Mœbius’s smallest comic book doodles grew to a starring role in Star Wars II.
Syd Mead didn't work on The Empire Strikes Back, but he made a significant contribution nonetheless, when his designs for US Steel ended up informing the imperial walkers deployed on Hoth.
Lucas didn't always see Vader as Luke's father; so how did that idea come into being?
Published so rarely that it's never actually been published yet.
May the force be with you.