It's as if they realized that audience weren't as enthused for this movie as they might have liked. The latest Guardians of the Galaxy trailer did very little to differentiate itself from the Avengers, except in space and with more irrelevant characters, and it was hard to see what the guardians really had to do with the rest of the Marvel Cinematic Universe. Was a two minute scene at the end of The Avengers really worth watching an entire two hour movie continuation of, when you have to be a real fan of the comics to even know any of the cast? Then new details were been released about the movie that make its place in the series a lot clearer than the trailers have.
The link is all in Thanos. The villain was introduced in the bonus scene at the end of The Avengers and director James Gunn has stated that he is going to show up in the Guardians of the Galaxy. We figured as much from the fact that Zoe Saldana is playing his green-skinned adopted daughter Gamora, but apparently she isn't the only one. Karen Gillan plays Nebula, who, in spite of appearing as an antagonist in materials so far, was revealed to have been raised alongside Gamora as her sister.
"They were bio engineered to become living weapons, so he's really good at killing people," Gunn said in an interview with Entertainment Weekly. "So clearly there's been a bit of a falling out between the two, since they're definitely having a pretty major fight scene."
As excited as it would be to see two kick-ass ladies go up against each other, I'm already hoping that Nebula and Gamora get to turn their relationship around before the end of the movie. There are more than enough adversarial female relationships in the media, so it'd be a lot more satisfying to have them recognize Thanos as their mutual problem and team up to take him down. Not that they'll succeed, since Gunn teased what we already know: that Thanos will be the main villain behind everything the Avengers deal with in each film.
Nebula is the main female villain of the film alongside Ronan, an admiral of Thanos played by Lee Pace, but it's not like villains can't be redeemed. Captain America spent his sequel fighting against the Winter Soldier as he murdered a lot of people and caused millions of dollars in property damage, but he's already started on his redemption arc. If the Winter Soldier can quite literally get away with murder in the Avengers universe, then why can't the same hold true for Nebula no matter what her role in the plot entails?
Yes, she's a supervillain in the comics, but she's not Gamora's sister in the comics. Changing that one simple thing could take her character in an entirely different direction. Nebula and Gamora may never be as close as Anna and Elsa from Frozen, but a girl can dream.