Forgot to mention that one of my favorite pieces of unused game artwork was the old Pac-Man artwork for the Atari 2600 by Hero Kimura. Atari rejected it because the ghosts' designs were deemed too scary by Atari.
...And it was rejected before it was even finished!
Hiro Kimura's interpretation of Pac-Man is a whole different game altogether, and it goes without saying that the artwork looks great, but in retrospect, while I don't think it would've been a big deal over in Japan, I'm inclined to believe that type of imagery wouldn't have been well-received in the States back then; I think had they gone with that as the cover, it would've done more harm than good. Or who knows, maybe it wouldn't have been a problem, and the takeaway would've been kids' disappointment over the game not being more like the cover art. The fact of the matter is that would've made for a badass cover. Period.
Anyway, I can't believe I completely forgot mentioning him in my original post—his art is incredible.