If the choice is Star Wars or Avatar getting the prime slot it would be Star Wars. They have Star Wars Land as well. And I doubt there is a single person who thinks Avatar Land is more of a priority for Disney than Star Wars Land.
This is what Weaver said about December 2018 a few days back:
http://www.slashfilm.com/avatar-2-release-date-delay/
At this point Star Wars will be pretty fully locked in as THE december franchise with 2015, 2016, & 2017. I could see Disney forcing Camerons hand here and schedule Star Wars against it to get Fox to blink and shift Avatar to Summer 2019 or along those lines. They will have owned December with Star Wars for 3 years straight by 2018.