Here's an interesting collection of rejected posters for the 1984 movie Supergirl, courtesy of Jay Allen Sanford for the San Diego Reader. Sanford scanned and archived these and many other rejected posters for a client, and it's fascinating not just because of "what might have been," but to see how films like Supergirl could have been marketed.
For example, here at the right we have a decidedly non-superhero, somewhat Victorian-romance version of the poster, complete with a flower and sappy tagline (the one on the left was kinda cool though):
Here's some more "non-super" designs, playing up the romance angle. Even the one on the left makes Supergirl look like she is seductively stripping rather than revealing her secret identity:
What I find interesting is how in some of these designs the iconic Supergirl/Superman comic book elements are played down in favor for more of a romance vibe. You'd think that the public would be familiar enough with the Superman mythos through the success of the previous films that this would not be necessary -- you'd think the concept would essentially "sell itself" due to it being branded as part of the Superman movie franchise. And yet, considering the disappointing box office of Supergirl, maybe it wasn't enough.
Lots more Supergirl posters at the link, plus rejected designs for other movies including Batman. And here's part 2 of the San Diego Reader feature, with posters for Howard the Duck and The Fly.