The producer Jon landau has talked about Avatar 2 commenting that what they want to do are independent films, with the same world in the background, so that the viewer, if he wants, can see them independently, not as in the sagas of "Star Wars" or "The Lord of the Rings".
“Well, to begin with, water will be part of the movie, but it's not going to be everything. There are many rumors that it will be an underwater movie, and it is not. It will be like the floating mountains, and how the Na'vi interacted with them. The mountains were part of "Avatar", with water the same will happen. We've barely scratched the surface of Pandora, figuratively and literally, so there are plenty of places we can explore. There is the ocean, but there are other parts of the world that we do not know yet, even some of the different clans that Jake recruited, so it is a very rich and varied world to explore.