Papa Vinyard here, now here's a little somethin' for ya...
We've had a good idea since 2010 that Sigourney Weaver's Dr. Grace Augustine was going to return in at least one of the upcoming three AVATAR sequels, but Ms. Weaver has seemingly just confirmed it herself. While chatting with Vulture, the ALIEN-killing, GHOSTBUSTER-seducing actress was quoted thusly:
"It's going to be an adventure! It will be awesome. I can't even imagine what I am taking on — and what James Cameron is taking on — with filming three AVATARs at once, but ... Peter Jackson did it with the HOBBIT films, so we have that to look to. It will be challenging for me. I can't talk about it, but my part is a little different in each one. I'll transform somewhat. We'll have to talk again when I survive this. For now, I'm going to skittle around and enjoy this leisure time."
Note how she says she'll be "different in each one." That roots out any one-off flashback appearance or anything like that. Even though she failed to link up with the Tree of Souls the first time around, I'm sure the goings-on at Pandora are such that resurrecting Dr. Augustine isn't out of the realm of possibility (reminds me of Max Landis' quote from his dad: "You can kill a vampire however the fuck you want because vampires don't fucking exist!").
Weaver brought an air of familiarity and professionalism to the unrivaled blockbuster, despite odd character choices like maintaining her smoking habit and her taste for Xanax hundreds of years in the future. 5 years later, and she's still easily the biggest name in the first film's cast, and I expect that'll remain such in the next films. With all the money going to CHANGING THE WORLD OF VFX AS WE KNOW IT, Cameron can't splurge on a Liam Neeson or someone, can he?
Now, who's going to explain why the Na'vi will bother to bring Colonel Quaritch back??
AVATAR 2, 3, and 4 will regrow the Hometree in December of 2016, 2017, and 2018, respectively.