Wonder Woman sounds right to us.
Like Superman, Wonder Woman is a character who has a ton of cultural cache, but that TV and film people have had a difficult time cracking.
That challenge should be the kind of thing that drives the script to greatness, and that appeals to the kind of filmmaker who could churn out a hit. Taking another look at Joss Whedon's script, which Warner Bros. presumably still owns, is probably a good first step but of course it's unlikely they'd be able to move forward on it without changes, especially if they want to connect it to the world of Man of Steel.
In any event, Wonder Woman is a the last piece of the Justice League puzzle, really. Everybody already knows who Batman is and what he brings to the table, and even if the Nolan version of the character is gone, there's not much in the way of a need to reintroduce him. In fact, so soon after a successful relaunch that focused heavily on Bruce Wayne, it may be better to see an almost exclusively-superheroic Batman, so bringing him into one of the other franchises rather than giving him a solo film before Justice League could prove to be just what the doctor ordered.
Wonder Woman, though, is a character that most moviegoers have only a passing awareness of. The tools of her trade--the bracelets, the lasso and the invisible jet--are as well known as the character is herself. She's a blank slate for the screenwriters, but at the same time neeeds to be defined for the post-Lynda Carter generation before a credible Justice League film can be mounted. If she's introduced in the film, Diana's origin will get the short shrift and fans will almost certainly upset a number of fans.
There's another thing that a Wonder Woman film would mean, though, if Warner Bros. were to announce it at Comic Con: they would beat Marvel Studios to a significant punch: the delivery of a female-led superhero film.
Besides the sociological importance of finally making a major superhero movie with a solo female lead--something eloquently dealt with in an essay at The Atlantic today--the idea of Warners being able to bring the first great superheroine to the big screen before Marvel gets one of her distinguished imitators on film would be a feather in the cap of a studio who's been routinely out-strategized and outperformed by their rivals.
After all, DC have Superman, Batman and Wonder Woman. As much as you might love Marvel, there aren't any characters in their stable that rival those three in terms of history, awareness and broad appeal outside of comicdom. That The Avengers will be the top-grossing cinematic franchise of all time by the end of the year is an indicator of just how much better Marvel have been at making their intellectual property work for them. Taking their successful superhero films to the next level with The Avengers, making the first billion-dollar superhero team film, there aren't that many options left for Warner to change the superhero-movie game.
Marvel, though, have struggled to spotlight their female superhero characters in spite of the success of the rest of their line. The only female hero to appear in The Avengers was Black Widow, who also appeared in Iron Man 2 and will be featured in Captain America: the Winter Solder...but excepting Hawkeye, there wasn't another Avenger that had less to do in the film. Her appearance in Iron Man 2 was expectedly shallow; after all, she's not the film's hero.
While it's arguable that Wolverine, Spider-Man and Captain America are on roughly the same level of public awareness as Superman and Batman, it's impossible to dispute that Marvel doesn't have a character equivalent to Wonder Woman. When it comes to the superhero genre, she stands alone in her class as the best-known and most marketable female hero--and that's why if there's going to be a massively successful, female-led superhero film, it probably starts with her. Anybody else would get fans and reviews citing Wonder Woman and Buffy the Vampire Slayer in all of the reviews anyway, so why not cut out the middle man?