This morning, in response to Rob Bricken's speculation about Disney buying the DC superheroes from Warner Bros., I pointed out that this almost happened... thirty years ago. According to former Marvel Editor-In-Chief Jim Shooter in a blog entry dated August 26, 2011, Bill Sarnoff, the head of Warner Communications' publishing division, approached Shooter in early 1984 about licensing the publishing rights of the DC Universe to Marvel, his rationale being that, considering the company's dominance over the comics marketplace, they were in a much better position to sell superhero comics than DC, which had been struggling in second place since the early '70s. Sarnoff saw the DC imprint as a dead weight and a consistent financial drain, and believed that the characters could thrive under Marvel's editorial and artistic guidance. After all, Marvel didn't own Star Wars, Micronauts, ROM, or G.I. Joe, but their licensed comic book series based on them had been hugely profitable for the company (and the properties' respective owners).
Had the deal gone through, the entire DC line would have been cancelled (remind you of anything else?), and Marvel would have started anew with seven titles, featuring the most famous and bestselling characters at the time: Superman, Batman, Wonder Woman, Green Lantern, Justice League, New Teen Titans, and Legion of Super Heroes, with plans to expand if the new series proved popular. (The Flash's standalone book was held over for the second wave.) The Marvel and DC Universes would have been kept separate, at least at first, and most of the existing continuity would have carried over, though some or all of the creative teams would likely have changed. (Several creators at DC, including Titans writer-artist team Marv Wolfman and George Perez, had left Marvel in the late '70s because they didn't like working with Shooter.) John Byrne would have drawn and written Superman (big surprise), and even drew a mockup of the cover for his first issue. A summary of Byrne's script can be found here. Shooter almost certainly would have written Legion — he'd always been fond of the series, not least of all because it's where he'd gotten his start as a comics writer at age thirteen.
The proposed deal never made it past the drawing room, though, since around this time First Comics sued Marvel over antitrust allegations. Buying out a chief competitor, or at least the publishing rights to their most recognizable properties, would not have looked good to jurors. But if it had gone through, the results would have been cataclysmic for a variety of reasons, and would have changed both the comics industry and popular entertainment as we know them today.
First of all, Saga of the Swamp Thing would have been cancelled just as the creative team of Alan Moore, Stephen Bissette, and John Totleben was hitting its stride and starting to get a lot of attention from the comics press. Today Moore's Swamp Thing is considered to be one of the seminal mainstream comics of the '80s, but it was a modest seller at best and generally thought of as a cult title at the time. If you were a middle-schooler back then you probably wouldn't spend your precious New Mutants/Transformers money on it, not least of all because your mom would probably freak out if she saw it. It's highly unlikely that Shooter would have given the book another shot as a "mature readers" series over at the troubled Epic imprint, which while "creator friendly" and "adult oriented," was infamous for censoring material before publication. Building up the DC brand at Marvel would mean focusing immediately on the biggest, most popular and accessible characters; a dark, Comic Code-free series about a philosophical plant monster and his chloro-curious human girlfriend would not have fit the bill. Without DC's support, it's unlikely that Moore would have found a wide audience in America. There would almost certainly have been no Watchmen. He might have done a series for one of the many creator-oriented independents of the era, like First, Eclipse (which published the first American editions of Miracleman), or Renegade, or he might have worked primarily for publishers in the UK. He certainly would not have had the same meteoric rise to comics superstardom, and might have given up on the medium altogether to focus on prose.
Without Swamp Thing, a bunch of late '80s/'90s series would never have come into existence. Hellblazer, Animal Man, Sandman, Doom Patrol, Preacher... basically the entire Vertigo line. An entire generation of British comics writers would likely have never crossed the pond. Grant Morrison and Neil Gaiman would have had very different careers, perhaps as journalists or television writers.
On an even more important scale, there would likely have been no Dark Knight Returns. Shooter, a staunch moralist (remember, it was his decision to make Jean Grey suffer or die for her actions as Phoenix), would have recoiled at the idea of a sixtysomething Batman who murders the Joker, Superman as a servile government stooge, and Selina Kyle as an overweight, aging prostitute in a Wonder Woman costume. He would have liked the idea of Miller doing Batman, but only in a traditionalist manner, similar to his work on Daredevil. Miller might not even have wanted to work with Shooter again; after all, he'd gone to DC because they'd promised him complete creative control over Ronin. Like Moore, he might have divorced himself from mainstream comics and gone to work for one of the independents. We could have gotten Sin City a decade earlier.
And without DKR, we'd be living in a much different world. Miller's intense grindhouse/noir aesthetic transformed the public image of Batman from a campy goofball to a brooding avenger almost overnight, a perception amplified by the enormous amount of coverage in mainstream media outlets like Rolling Stone and The New York Times. (This led also to the proliferation of the "Bang! Pow! Comics Aren't Just For Kids!" meme, which helped to bring mainstream attention to other non-superhero comics like Maus and Love & Rockets.) Finally, the popularity of DKR spurred Warner Bros. to fast-track the long-in-development Batman feature into production. The 1989 Tim Burton film's huge success not only cemented the "comic book movie" as a blockbuster category, it also changed the way big budget movies were sold to the public, with an emphasis on brands and franchises rather than stars or stories. Without Miller's miniseries, there would be no Burton Batman, no X-Men, no Spider-Man, no 300, no Dark Knight, no Marvel or DC Cinematic Universes. We might have had adaptations of one or more of those properties, but they would be informed by very different sensibilities, like a Spielberg-produced, Joe Dante-directed Batman based on the Pop-Art style of the 1960s show, or a James Cameron Spider-Man that revamped the story as a Michael Crichton-type technothriller to make it more accessible for non-comics audiences. Without the aggressive "logo" campaign of the first Burton movie, the marketing of tentpole movies would be completely different today. We might not even talk about sports team-style "franchises" the way we do now — they might still be making Bond or Star Trek movies, but not films based on toys or amusement park rides. Without Batman, there would be no model for pitching those kinds of movies to audiences.
In the meantime, the comics audience would have probably gotten more insignificant and more nichefied over the years, as it has in real life, though the focus would be different, with companies aiming their wares at younger viewers in the hope of selling cartoons and toys (sans $150 million movies). There would probably never have been a Miller/Moore-influenced "grimdark" phase, at least in mainstream superhero comics. (Adult comics would exist, but only as independent fare like the Fantagraphics line, Chaykin's American Flagg! and Sim's Cerebus.) The publishing industry would also be different. In the late '80s and early '90s, the Warner-published trade paperbacks of Dark Knight Returns, Watchmen, and Sandman were instrumental in getting comic books into the hands of people who would never venture into a comics shop. Without them, the concept of "graphic novels" as a publishing category would likely have remained obscure, limited mostly to European material or small presses. Marvel would likely have ended up buying the rights to the DC characters wholesale, though the resulting fusion would not be a unit in a global megaconglomerate. Without the prospect of mainstream exposure, or the glamour of functioning as an R&D lab for billion-dollar licensing deals and multimedia franchises, superhero comics would have likely become more hermetic over time, even more so than today. There would still be retailers and conventions, but Comic Cons would be minor, oddball affairs that would only get attention from the local presses, with an emphasis on "look at the weirdos in spandex"-type copy. Lots of cartoonists would have avoided the industry altogether, selling their work on the Internet. In effect, if Marvel had taken control of the DC characters in 1984, the comics industry would probably look the same today as it did in 1984... just smaller, more expensive, and more self-absorbed.