Sure, movies like Iron Man and The Dark Knight have proven that superheroes can work on the big screen, but sometimes only comics can offer longjohned epics so large-scale that they'd break Hollywood in half through special effects budget alone.
Few remember - and maybe with good reason - the end of Marvel's late-'80s experiment, the New Universe, in which a man driven mad with seemingly-unlimited power accidentally turns Pittsburgh into a radioactive crater by trying to get rid of his powers, and the world goes to hell from that point onwards: America becomes filled with irradiated monsters, nuclear war and godlike children who demand that we make war no more, or else. A weird and forgotten piece of post-Watchmen superheroics, but one that mixes old-school and new-school with an admittedly naive worldview that still may be too big for one movie.
One of comics' first as-many-superheroes-at-once extravaganza, it's not just the idea of bringing the Avengers, X-Men, Fantastic Four and random other characters (Spider-Man! The Hulk!) together to battle an army of supervillains that might make this colorful story too expensive to film, but the creation of a patchwork planet for them to fight on, along with the various alien races and/or technology that they meet along the way. Then again, the comic was created to support a 1980s toyline, and we all know how well Transformers and GI Joe have done for movie studios, so perhaps we should never say never...
This year's strange superhero disaster movie killed off a number of Marvel's more popular characters, amongst them Wolverine, Thor and Doctor Doom - but only in their alternate timeline, "Ultimate" incarnations. Nonetheless, the story - in which Magneto causes all manner of "natural" disasters, destroys New York and decimates the superhero population of the planet before finally being stopped by a mix of X-Men and other superheroes - is pretty much 2012 with added superheroes, the idea of which may be the ultimate (No pun intended) high concept, but the cost of which would make most effects budgets weep.
DC's 1988 crossover is ID4 meets Star Wars, and then some: Different races of alien invade Earth to find out why the planet keeps producing so many superheroes, in the process destroying Australia (This is back when you could do that kind of thing without people getting upset that you've killed off an entire continent), performing genetic experiments on normal humans to see if there's some latent superhero gene (There is) and fighting a war on two fronts, as Earth's superheroes defend their planet with the help of some turncoat aliens. With a cast that's about 50% alien (And multiple types of alien, at that, with only a couple achievable with Star Trek-esque nose attachments), space battles and all manner of high-scale superheroic takes on your favorite war movies, this would be a sfx extravaganza... If anyone would ever be able to afford it.
Flipping between "reality," imaginary worlds, time periods and everywhere in between, Flex may just be one of those unfilmable projects even before you start to think about how much it'd cost to have an army of superheroes destroy a city, combine to form a new reality that we live in, or even just have the orgy that proves Frederic Werthem right. But factor in the need to create surreal fictitious cities for the young Wally to get lost in, atomic explosions and mutations or even just costumes to match Frank Quitely's awesome fashion sense, and you're left with the kind of movie that would need Watchmen-esque precision and care, but for an even-less mainstream audience friendly story.
DC One Million
Again, just the scale of work needed to bring this story to life would make most people in charge of budgets get nervous: Taking place in two different eras (Today and the 853rd century), on multiple planets and with large-scale destruction brought about by a nano-technological virus that comes from a living robotic sun, the necessary design process alone would probably scare off movie producers before it even came to the idea of making it all look convincing. A cast of hundreds of superheroes from both eras (Including a Superman from the future who has to sparkle, just like Twilight's Edward) would just add to cost woes.
Crisis On Infinite Earths
Talking of casts of hundreds of superheroes, DC's 1985 big daddy of all superhero crossovers is the kind of thing that would have to be told in a series of movies, and even then would still be missing all manner of greatness: Requiring multiple Earths to be created just so they could be destroyed, taking place in multiple time periods - including a part of the story where the time periods merge together so we get to see World War II fighter planes fight dinosaurs - and with almost every character in the story (and there really are hundreds) a superhuman and requiring some level of ridiculous costume and special effects to be made real. While it might not be the greatest comic ever (Or even the greatest superhero comic), this might be the most perfect example of a story that is too full of ideal comic book imagination and spectacle to ever make it to the silver screen.