Avatar released in 2009, and the furthest thing from my mind has been the, not one, but three Avatar sequels beings made. James Cameron is shooting the scripts simultaneously and then releasing the three sequels in three consecutive years.
Unfortunately, the timetable for these releases have been pushed back. Avatar 2's release date has already been pushed back a couple of times; now, Cameron is saying that the former December 2016 release date for Avatar 2 was too ambitious. The film was pushed back to late 2017, mainly due to the writing process being "very involved."
ABC News reported this quote from Cameron:
"There's a layer of complexity in getting the story to work as a saga across three films that you don't get when you're making a stand-alone film. We're writing three simultaneously. And we've done that so that everything tracks throughout the three films. We're not just going to do one and then make up another one and another after that. And parallel with that, we're doing all the design. So we've designed all the creatures and the environments."
So as of now, we have Avatar 2 coming in late 2017, Avatar 3 coming in 2018, and Avatar 4 in 2019.