Disney recently announced it had delayed the release of Star Wars: Episode VIII until December 15, 2017 - which has now had a major affect on the sequel to Avatar, also due in Christmas 2017.
According to The Wrap, 20th Century Fox has now delayed Avatar 2 - even after James Cameron said they were targeting the holiday release window of 2017. But, the release of Avatar 2 has been up in the air for a while, rumored for 2014, 2015 and even 2016 - but now, who knows when James Cameron's next juggernaut will be released.
But, it makes sense that they wouldn't want to compete against Disney and the unstoppable Star Wars franchise, especially during the holidays. We're not just talking about the movie here either, but merchandise, and everything else that launches with Star Wars: Episode VIII, and Avatar 2. We should hopefully see Avatar 2 released in 2018, but at this rate it might be 2020 before Avatar 2 is released.