The Nightmare Before Christmas is just one movie. Released over 30 years ago. Still ENORMOUSLY popular with a huge cultural impact. Now, if someone said they were building a Halloween Town land at a Disney park, the TNBC fans would go absolutely WILD about that news. (And honestly, as someone who just thinks it's a cute movie and doesn't understand the longevity, I think it would be a great idea for it to have it's own attraction at the very least and stop hogging my Haunted Mansion.) It has a huge presence at the parks, especially around the holidays, but even all year round with the merch that's always for sale.
So, yes, Avatar has made a lot of money, and apparently it does have some devoted fans (you literally just named a character I've never heard of, so I am guessing you might be one of them). But it's really not a stretch to say it doesn't have the cultural impact as many other films/TV shows that are out there, franchises or not. And it's no offense to those who love it, but the rest of us mostly remember the visuals and not the characters or the story.
That said, if they do an absolutely spectacular job at making Pandora or whatever they're going to do at DLR, I'll be the first to say so for sure. Cars Land is so brilliantly done that while it's not even close to the top of my favorite Pixar or Disney movies, it's a A+ experience. So, while a lot of us may not be excited about Avatar coming to the parks, if they do as well with it as they did Cars Land, then that's a wonderful thing.