The Justice League Comic-Con trailer has sparked a lot of conversation and speculation!
Now the latest fan theory is that Supergirl is in the Justice League Comic-Con trailer, with the thinking that Supergirl is somehow tracked down to replace Superman in the Justice League (one theory). If you recall during Man of Steel when Clark found the ship, one of the pods was opened. An official digital tie-in comic book revealed it to be Supergirl (not sure if it's canon though), so it's possible Supergirl could exist somewhere in the DCEU.
The Supergirl speculation (via reddit) comes from a particular scene in the Justice League Comic-Con trailer with Ben Affleck looking at something that resembles Superman (or Supergirl). The current thinking is that it's some sort of hologram. My guess is it is Superman, and it's a homage, memorial or reminder of Superman.
However, it could very well be Supergirl. The voice over at the time of the scene is from Steppenwolf who says, "No Kryptonian." That could mean any Kryptonian. It's safe to say that Darkseid and Steppenwolf have had run-ins with the Kryptonians before, just by the simple fact that Steppenwolf also says, "No Lanterns," which means the Green Lanterns exist somewhere in the universe, just like the Kryptonians once did.
Here is the screenshot from the Justice League Comic-Con trailer, where here to me, it looks like Superman:
However, here is where the Supergirl speculation comes from:
I'm also guessing the Supergirl speculation comes from what almost resembles woman's leggings of some sort as whatever those markings are go up to the thigh:
I'm not exactly sure why the hologram (or whatever it is) features Superman's (or Supergirl's) legs that way, but maybe because it is a hologram or some sort of projection, it is needed to make it look 3-dimensional.
Now the question I have for the Supergirl theorists out there: Where has Supergirl been? If you also recall during Man of Steel, the ship crash landed thousands of years ago, and the Supergirl prequel comic confirms this as well. So does that mean Supergirl has been alive on Earth for thousands of years?
I actually might have the answer, and it's no.
The end of the Supergirl Man of Steel prequel comic book saw Kara in the the captain's chair, and one of the last things the computer emits is: "initiating self-repair!" The comic then goes to the present day where Mission Control discovers a signal from Canada, and we then see Clark Kent on the fishing boat heading there, which is continued in the Man of Steel movie.
So it's possible that it may have taken thousands of years to "self-repair" Supergirl (due to the crash landing) and the pod opened up in the present day and then emitted the signal.
Regarding why Supergirl hasn't been around until the Justice League movie? She hasn't had her powers or been at full power. She's been buried under the ice for thousands of years and hasn't been exposed to the sun. It's possible she went into hiding or maybe was even found by somebody?
Obviously this all sounds pretty far-fetched, but it could be plausible, and Supergirl has been rumored for Man of Steel 2. Maybe Supergirl debuts in Justice League?
(fan art via SilentArmageddon Deviant Art)p
"Justice League" has a November 17, 2017 release directed by Zack Snyder starring Ben Affleck as Batman, Henry Cavill as Superman, Gal Gadot as Wonder Woman, Jason Momoa as Aquaman, Ezra Miller as The Flash, Ray Fisher as Cyborg, J.K. Simmons as Commissioner Gordon and Willem Dafoe an Atlantean, Nuidis Vulko.
Fueled by his restored faith in humanity and inspired by Superman’s selfless act, Bruce Wayne enlists the help of his newfound ally, Diana Prince, to face an even greater enemy. Together, Batman and Wonder Woman work quickly to find and recruit a team of metahumans to stand against this newly awakened threat. But despite the formation of this unprecedented league of heroes—Batman, Wonder Woman, Aquaman, Cyborg and The Flash—it may already be too late to save the planet from an assault of catastrophic proportions.