Zack Snyder's adaptation of one of the most famous comic books of all time took a few liberties, but an earlier film adaptation plan would've been much different.
Snyder's 2009 film version of Alan Moore and Dave Gibbons' Watchmen was, in many respects, quite faithful to the story that many readers have deemed one of the greatest comics of all time. Many of the comic's iconic images and character moments were preserved for the big screen, but one big deviation stood out: the ending. In the film, the mastermind at the heart of the story's major conspiracy was still Adrian Veidt (Ozymandias), but instead of depicting him teleporting a tentacled monster into New York City to simulate an alien invasion, the film showed Ozymandias engineering similar destruction around the world through energy reactors, then pinning that destruction on Dr. Manhattan. The goal -- to unite the world against a common enemy, and therefore prevent nuclear war in the 1980s -- was the same, but rather than an engineered alien menace, Veidt placed the blame on Manhattan's shoulders.
Though the outcome was essentially the same, many fans of Watchmen in its comic-book form would consider that a major change. It's nothing, though, compared to what might have been had the Watchmen film been made a bit earlier. According to producer Joel Silver (The Matrix, V for Vendetta, Sherlock Holmes), who was attached to produce the film in the 1980s, writer Charles McKeown and director Terry Gilliam had a very different solution for the film's third act, one that still involved Dr. Manhattan but also brought time travel into the picture. In a recent interview, Silver described what would've been a very different Watchmen.
"What Terry had done, and it was a Sam Hamm script–who had written a script that everybody loved for the first 'Batman'–and then he brought in a guy who’d worked for him to do work on it [Charles McKeown, co-writer of 'Brazil']. What he did was he told the story as-is, but instead of the whole notion of the intergalactic thing which was too hard and too silly, what he did was he maintained that the existence of Doctor Manhattan had changed the whole balance of the world economy, the world political structure," Silver said. "He felt that THAT character really altered the way reality had been. He had the Ozymandias character convince, essentially, the Doctor Manhattan character to go back and stop himself from being created, so there never would be a Doctor Manhattan character. He was the only character with real supernatural powers, he went back and prevented himself from being turned into Doctor Manhattan, and in the vortex that was created after that occurred these characters from 'Watchmen' only became characters in a comic book."
So, rather than engineering a situation in which the world would have to unite to face some kind of common threat, in this version of the story Veidt simply took out the paradigm-shifting character of Dr. Manhattan altogether. Apparently this would've not just changed the economic and political climate of the world. It also would've made the rest of the Watchmen heroes irrelevant and, therefore, sort of fictional.
“So the three characters, I think it was Rorschach and Nite Owl and Silk Spectre, they’re all of the sudden in Times Square and there’s a kid reading a comic book," Silver said. "They become like the people in Times Square dressing up like characters as opposed to really BEING those characters. There’s a kid reading the comic book and he’s like, 'Hey, you’re just like in my comic book.' It was very smart, it was very articulate, and it really gave a very satisfying resolution to the story, but it just didn’t happen. Lost to time [...] But I did like the  movie, very much. Zack did great stuff in it!”
So, if we'd seen this earlier version of the film, we would've definitely gotten an even bigger departure from the source material. We'll never see this Watchmen on the big screen, so it's hard to guess exactly how it would've been received, but one thing's for sure: We want to visit the alternate universe where Alan Moore gets to react to this version of the movie.