If Cameron ends up postponing the film for even more years, 2020 could be good for him, as it would come the year right after Episode IX and Infinity War Part II. For me what's disapointing with James Cameron is that he's losing the opportunity of making more movies, i mean, i know he's helping push technology in film forward, but i wish he to have seen some more films from him during the 2000s period, and even in the present.
He also has some other interesting projects planned, so i hope he doesn't spend half as many years making them as he seems to be doing for the Avatar films.