As for Avatar, yes, it was a big hit in 2009 -- that is, until the next big thing came along.
And that "next big thing"? Disney's very own "The Avengers"!
I would much rather see Disney Parks capitalize on the mega-money properties they already own such as "Star Wars" and "The Avengers" (by, perhaps making a "swap" with Universal of "Avatar" for the rights of the Marvel characters they are currently using?)
Likelihood of any "swap" would be pretty low. Cameron would have to agree, too...and I'm not sure that would be in the cards. Disney couldn't just simply transfer the existing contract over to Universal.
Hypothetically, even if he agreed...it would take more than a straight swap. Marvel Island is already built. Uni would want the money they spent to build it, the money it would cost to dismantle it, and the money it would take to build something else as WELL as a hefty penalty/premium to convince them to get out. The Avatar swap might go, slightly, toward that premium..but you're probably still talking about close to a billion in cash to go along with it to "convince" Uni to turn the Marvel rights back over to Disney.
And the alternative is that Disney sits back and collects checks from Universal, basically doing nothing, investing nothing, and taking no risk.
On the flip side, Uni has a popular area of their theme park, which brings in customers (and whose attractions are considered highlights of the park), and which is making them substantial money. Other than an obscene amount of cash...they really have no reason to change things out. It would require them to shutter a good sized section of their park, spending more money on demo, design, and rebuild, all while taking a substantial risk that what they build isn't as popular as what they have, right now.
I don't see much reason for either side to rock the boat, as it were.