Comic book films have had an interesting cinematic journey. They were basically considered smut - nothing but low-brow entertainment for the lowest common denominator - in their infancy, and that attitude persists today in certain circles.
Over the years, though, something happened: they went from being the laughing stock of Hollywood, to getting nominated for Oscars and dominating the box office at every turn. Now, that's not to say that every superhero film of the 21st century has been a cinematic masterpiece. Comic book cinema has been a mixed bag. We've gotten (very) good and (very) bad, and everything in between. For every Logan, The Dark Knight, and Captain America: The Winter Soldier, we had to endure a Jonah Hex, Fantastic Four, and *shudder* Catwoman.
All that to say this: the following list won't be an examination of just how consistent superhero films have been. They haven't. Not in the slightest. We won't be making a case for how much better or worse off the superhero genre would be if these films made it to the big screen. It's had - and still has - a pretty wide range of quality without these films influencing it one way or another.
No, these are just the films that could have been. Some for better, some for worse, and some we're not quite sure of.