It's weirdly fitting that The Dark Knight and Watchmen look like they're going to end up serving the same purpose for movies that The Dark Knight Returns and, well, Watchmen served for comics twenty-plus years ago; namely, misdirecting creators into thinking that superheroes have to be "dark" and "realistic" in order to be successful. There's every possibility, of course, that Watchmen's theater bow will end up far lower than expected (That 78% drop this Friday doesn't look good, let's face it) and shift this expectation - just as there's the possibility that Warners and Fox execs are right in assuming that the mainstream movie audience really does just want grumpy, moody superhero movies. But somehow, I doubt both of those possibilities.
That Hollywood wants another Dark Knight is completely understandable, even ignoring the exceptional box office take of the movie; Chris Nolan's second Batman flick was artistic, sincere and adult (even if it was also twice as long as it should've been), and showcased a new way to approach superhero movies without traditional sensationalism and cliche, after all. But that doesn't mean that it needs to become the template for all superhero movies from now on. Part of what made Dark Knight work so well was that the darkness and the stabs at moral ambiguity were entirely in keeping with the world of Batman as we've all come to accept it in recent years. It wasn't an attempt to graft something that didn't belong onto the character - as, I think could be argued, Superman Returns tried to do with the "Lois has moved on/Clark is a stalker" angst that hit such a false note - but an indulging in the greyness that Batman has become increasingly surrounded in since his day-glo '60s TV show heyday.
(The same with Watchmen's grit, which was always in the source text. In fact, there's definitely an argument that what makes Watchmen unsuccessful to some degree - and, of course, there are all manner of arguments over just what that degree may be - are the deviations from the original, whether it be in plot of the slick glossiness that Snyder's movies seem to have no matter what.)
The original two Fantastic Four movies weren't financial failures, per se, but they weren't Spider-Man or X-Men, either; the reason for that, though, wasn't that they were too happy. Creatively, they were... fun enough in a "I'm half-watching them on an airplane because there's nothing else to do" way, perhaps, but they weren't really the Fantastic Four, either. The key to those characters and that series is, ultimately, still in the tone that was established in the first 100 issues of their comic - a strange mix of sentiment, soap opera and invention that, to be honest, is much closer to the Back To The Future movies (or, more obviously, Pixar's The Incredibles) than Fox's previous attempts... and something that's a million miles away from the "dark" reinvisioning that rumors are suggesting that Fox is considering.
(The mind boggles, in a way, at the idea of a dark Fantastic Four franchise; the characters are so amazingly undark - they're called the Fantastic Four and their main nemesis calls himself Doctor Doom, for the love of God; there's a childlike suspension of disbelief necessary to buy into the concept as something other than parody in general, surely? - that I can't really imagine how it would even work for an entire movie never mind the reinvention of a franchise. If it is being seriously considered, it speaks to the assumption that it's a result of desperation to keep movie rights than anything else.)
The key to successful superhero movies, ultimately, is in faith to the spirit of the original (Case in punny point: The Spirit may have shared character names with Will Eisner's comic strip, but little else, and was an artistic failure because of that. Of course, it was a financial failure because it was a terrible movie), but that point - maybe the real lesson of The Dark Knight, especially if taken in tandem with last year's also-successful, but much more playful, Iron Man - is perhaps lost behind the dollar signs that appear in movie executives' eyes whenever Dark Knight is mentioned, particularly in light of all the publicity push of the equally gloomy Watchmen. It's a shame, because the superhero genre - for all its faults - is something with so much more potential than any amount of gritty deconstructionist, "realistic" takes could properly demonstrate, and various characters already headed to the big screen - whether they're Green Lantern, Thor or even Captain America - deserve stories that are much, much bigger than real life.