Hey Tommy, welcome! Just as sound and colour were are gimmicks, neither is 3D. When Hollywood uses 6 week post processed 3D for the sole purpose of grabbing, maybe then you could call it gimmick. Avatar though, that's a different story as it was filmed in native 3D for the purpose of delivering something truly beautiful and 2.78 billion dollars says it succeeded in doing so.Avatar had many more draws than 3D, it has a compelling love story and is character driven, the beautiful Pandora that took so many people's breath away (in 2D aswell!), action and science fiction.
Cameron doesn't do gimmicks, he's an artisan who works to improve the craft, however if I were to name the new breakthroughs for Avatar 2 this will be my predictions. Dynamic frame rate, Jim (James Cameron) calls frame rate a tool rather than a format, he will play action scenes in higher fps. Underwater motion capture, this has never been done before and in Cameron fashion he has created a new technology to bring his vision to life, except the best underwater visuals of all time (step aside Blue Planet 2 (they used Jim's cameras anyway)).