I don't think it will flop but you're right the first one was the hit it was for every reason other than the movie itself. It was the apex of new-age 3D theatrical experiences, a wave that propelled even crappier movies (like Tim Burton's first Alice in Wonderland film) to billion-dollar status. Avatar was and remains the best use of that technology. Seeing the movie was about the experience, not about the movie. Thankfully, the 3D fad has mostly died out. Unless Cameron has something groundbreaking to go along with Avatar 2 (and he doesn't or he'd already be working with theaters to implement it ahead of the release) the movie will be just another franchise sequel. It'll do good money, but not earth shattering money. I expect around between 750 million to 1 billion, depending on if the story is better this time around.