Avatar had a lot of box office dollars, but is nowhere near the obsession HP is. People play quidditch, for crying out loud.
It didn't just have "a lot of box office dollars", it's the #1 grossing film of all time. By a wide margin. (Not adjusted for inflation, of course.)
So clearly someone--a lot of someones--liked it. And with 2-3 film sequels in the works and other possibilities (cartoon, comics, books, etc.), it's bound to remain in the public eye for years to come.
No, it probably won't ever have the lasting appeal of Harry Potter but that ship has sailed. HP is no longer an option.
And in terms of bringing a new audience into Disney's theme parks, I would say Avatar has more potential than either generic offerings (Beastly Kingdom, Australia) or most owned content that hasn't yet been exploited (Incredibles, Up, Wall-E, etc.)
In addition to a lack of concept art I've heard there are 'creative differences' between imagineering and Cameron.
I think we've all heard variations of the same stories. What we don't know is:
1. Does a rift truly exist?
2. How serious is it?
3. Could it keep the parties from working together?
Disagreements happen all the time in the business world--particularly when creatives are involved. And Cameron already has a reputation for being difficult to work with.
That doesn't mean they won't ultimately find common ground and move forward.