Sigourney Weaver died in Avatar (I honestly had to look that up) but she's coming back for the three sequels. How? James Cameron sort of explains in a statement released by Fox:
"Sigourney and I have a long creative history, dating back to 1985 when we made 'Aliens.' We're good friends who've always worked well together, so it just feels right that she's coming back for the 'Avatar' sequels. Her character of Grace Augustine, as fans know, died in the first movie, so she's playing a different and in many ways more challenging character in the upcoming films. We’re both looking forward to this new creative challenge, the latest chapter in our long and continuing collaboration."
A different and more challenging character? I'm guessing it's a version of Eywa, the life force in the Na'vi trees (I had to also look that up). Like maybe her human identity merges with the planet or whatever New Age bullshit they come up with, but basically she's a Force Ghost.
The most interesting thing to me is that this announcement makes it seem like the movie is still headed towards a December 2016 release, placing it square in the same period as the first Star Wars spin-off movie. Some of us aren't confident that Avatar: The Quickening will make its stated release date, but if it does this tussle of the titans will make Batman vs Superman vs Captain America looks like a fart in the wind.