The way it profiles, there's going to be Avatar 2 in 2019, Avatar 3 in 2021, Avatar 4 in 2023 and Avatar 5 in 2025.
Cameron says this gonna take him about 8 years, so it figures. I didn't believe 2018, was on with a filming start in mid 2017, 18 months is too short due to the technical difficulties involved.
There's also the possibility that our dear Jim will want to make round years and push Avatar 3 in 2023, Avatar 4 in 2024 and Avatar 5 in 2025.
I think this last scheme might be on, why? Because once 2 is out in 2019 they gonna spend a year promoting it. So they won't be able to complete work fast for the rest. However, once 2 is out and done, finishing the others might be done by 2022/2023, and then they will be free to promote the others as they come out year in year out.