Cameron is 62 and has 4 movies in pre-production who will be released (supposedly) in 2018, 2020, 2022 and 2023. Most parts of these movies are CGI with motion capture performances so that could go quite quick but still it's a huge task. Most assets will probably be re-used for these movies (character models, textures, etc).
Avatar 2 will probably see december 2018 (lets pretend it won't be delayed again) release. That year it will go up against (not sure what months) Star Wars Han Solo, Pacific Rim 2, Avengers: Infinity War, The Predators, Black Panther, Ocean's 8, Deadpool 2, Jurassic World 2, The Incredibles 2, Aquaman, Mary Poppins Returns, Fantastic Beasts and where to find them 2 and more. That is quite a competition. I'm betting on a flop. But the themed area and it's ride could be amazing, irrelevant but amazing.