There are a few differences between Mysterious Island and Avatarland that I think need to be pointed out:
(1)Yes, Mysterious Island is based on a novel that is over a hundred years old. As a result, the public is more likely to view it as "original" simply because it hass not been in the public consciousness recently and most people have no predetermined opinions about it. Avatar is still pretty fresh in everyone's minds (even if a lot of people have forgotten about it when compared to Potter or Star Wars), and people certainly still do have opinions about it (as seen quite readily in this thread).
(2) Mysterious Island was originally funded by the Oriental Land Company for Tokyo DisneySea and NOT The Walt Disney Company. There is a huge difference between what OLC is willing to pay WDI to do and what WDW is willing t pay WDI to do. If WDW was willing to pay for a copy of Mysterious Island, I'm sure it would already be designed, built and operating at this point.
(3) The creator of the stories featured on Mysterious Island, Jules Verne, is long dead. James Cameron is very much alive. As such, Disney could have interpreted Verne's stories however they wished without the imput of the creator. Cameron, on the other hand, is very much involved and, apparently, has a lot to say about how Disney uses his property. This alone changes how, what and if Disney will do with Avatar, and, in a way, limits the design possibilities much more than Mysterious Island. (And by limited, I mean limited by Cameron's demands vs. Disney's willingness to pay, not a limitation of WDI's ability to recreate Avatar in physcial form.)