Only thing I think is that the Avatar sequels are taking way too long. They would have done gangbustsers 3-4 years ago. They will still do well today, but we are talking about a $2 billion plus movie that unlike Star Wars, Jurassic Park, and some other franchises has lost a lot of its luster. No one is buying avatar merch, humming its score, or even rewatching it today with reverence like they are others. Enough time has passed that people are starting to forget about it, and worse has become more known for being a Fern Gully or Pochahontas "rip off".
So once Rogue One finishes it's run the all time domestic top 10 will look like this:
1. Star Wars: The Force Awakens - 936 mil
2. Avatar - 760 mil
3. Titanic - 658 mil
4. Jurassic World - 652 mil
5. The Avengers - 623 mil
6. Star Wars: Rogue One - 550-600 mil
7. The Dark Knight - 534 mil
8. Finding Dory - 486 mil
9. Star Wars: The Phantom Menace - 474 mil
10. Star Wars: A New Hope - 460 mil
4 out of the top 10 all time will Star Wars films. Insane. No other franchise has more than 1 in the top 10. Over the course of just December '15 through December '16 Disney had 3 films that cracked the top 10 of all time.
Next year the only two films with realistic chance of cracking the top 10 are also Disney films, Star Wars Episode VIII and Beauty and the Beast.
Avatar was a novelty, built not on lasting characters or settings but on fancy new technologies. Popularity dropped off fast on that one. I don't see a sequel resonating with people the same way the first one did if people remember it at all.