I'd much prefer a Marvel park over an Avatar one, Marvel has franchises within the general franchise to build on. Avatar has two environments and very limited characters you could use outside of a dark ride. Plus, while it was a big movie with 2 more on the way, I don't think Avatar has the sticking power that Marvel has, people can associate with superheroes - they'll visit for their favourite one - but aside from the 'it's a new park' novelty, personally I wouldn't visit something Avatar themed.
Also, if it is such a massive project then they're likely to end up drawing guests away from DLP and WDS and having a very busy Avatar Park. Similar to the effect Wizarding World of Harry Potter had to Islands of Adventure, that area is jam packed and Forbidden Journey (rightly so!) has a 3 hour queue, where Spiderman/Hulk/Jurassic Park struggle to get 20 minutes! It's a dangerous game to play when you have a brand as big as this!