Some are already under development, such as Y: The Last Man, Fables, Preacher, Scott Pilgrim, and ElfQuest. Here is my list of some of my favorite graphic novels that should be given the live-action adaptation treatment. (Note, I'm not including comics that I think are impossible to do without animation, like Maus or Bone.)
Batman: The Dark Knight Returns - Frank Miller's harsh, brooding, but imaginatively dystopian portrayal of the Caped Crusader is a high point in the history of comic books. Like Watchmen, many wonder if it could ever be properly adapted, but I say give it a shot. An old Batman coming out of retirement, a twisted Joker battling his nemesis one last time, a female Robin, a Superman who is a puppet of the goverment, a one-armed Green Arrow, and much more, all add up to an epic, mythic superhero adventure. (By the way, forget the sequel The Dark Knight Strikes Again -- the art is nowhere near as good and the story is a lot weaker and often insulting to fans of the characters.)
Kingdom Come - This is one of my favorites -- writer Mark Waid explores the future of the DC Universe through an engaging tale that includes older versions of all the classic heroes. The highlight is a battle between Superman and Captain Marvel, and the climax is a nuclear detonation in the heart of Kansas. The artwork by Alex Ross is stunning. The casting director for any movie version should use his drawings as guidelines for the types of actors that would do the story justice.
Marvels - Speaking of Alex Ross, here's another one of his visual masterpieces. Written by Kurt Busiek, it shows iconic moments from Marvel history through the camera lens of a photographer. A movie version would be an interesting examination of the superhero genre from the point of view of the average citizen (and the audience).
Transmetropolitan - Warren Ellis has written some incredible comics, including The Authority and Planetary, but Transmetropolitan, with art by Darick Robertson, is my favorite. Spider Jerusalem is a journalist in a post-apocalyptic world, returning to society to face all the dangers and injustices he had left behind. Apparently, the movie rights to this story have been sold, so let's hope they eventually make a half-way decent film out of it.
Superman: Red Son - What if Kal-El, the last survivor of the doomed planet Krypton, landed in the Soviet Union instead of the United States of America? Writer Mark Millar and artist Dave Johnson explore the possibilities. Warner Bros. is looking for a fresh angle on the Man Steel to revive the franchise. This would be it.
The Sandman - Neil Gaiman has given us some amazing fantasy stories, some of which have already been made into great movies. The Sandman saga is probably his best work and I would love to see a good director tackle the challenge of adapting it.
Crisis on Infinite Earths - This is the groundbreaking mini-series by writer Marv Wolfman and artist George Perez that took DC Comics' decades-old convoluted continuity and tried to streamline it. I don't think a faithful live-action adaptation can ever be made, but it would be fascinating to see Warner Bros. and DC Entertainment use a similar premise to explore its live-action adaptations -- all the different actors still alive who played Batman and Superman, the Flash from the TV show, Linda Carter's Wonder Woman, drawn together to battle a cataclysmic, alternate universes hopping crisis, paving the way for a new era for the heroes of DC.
Lost Girls - Alan Moore's tale of grown-up Alice from Wonderland, Dorothy from Oz, and Wendy from Neverland is definitely for "mature audiences" but it is a marvelously written story. Moore himself calls it "porn" but in the hands of a competant filmmaker, and a brave movie studio, it would make an interesting movie. Too bad Stanley Kubrick is no longer with us, because it would be interesting to see what he could have done with this.
I'm sure you all can recommend others. I didn't mention some graphic novels that have generated lots of praise but I never read, like Ex Machina, Fun Home, Asterios Polyp, Blankets, or Black Hole. If there are others I should check out, please let me know.