Avatar doesn't strike me as a franchise that will enjoy the kind of tireless fanbase that surrounds properties like Harry Potter and Star Wars. The fandoms for those two franchises maintain enthusiasm even when there are no new projects on the immediate horizon. Fans read the books over again, watch the movies again and share them with their children, and discuss them online, produce fanfiction and fanart, organize meetups at conventions, etc. Star Wars is easily a cultural staple, and Harry Potter seems to be on the path to that status if one were to argue it hasn't been around quite long enough yet.
Avatar may have its enthusiasts, but it's not something I see shared and reblogged often. Or... ever, really. Maybe the new movies will stoke interest, but I'd be very surprised if its fandom generates life enough to last years beyond the new releases.
That said, I think the park itself will be a lovely experience. I wish it weren't connected to Avatar, though. I think it'll feel dated in the future in a way that Star Wars and Harry Potter's parks won't. It's difficult to imagine it as a story that will continue to feel fresh by merit of enchanting multiple generations, as so many of Disney's movies have accomplished.