Sky Fortress will be the first of three major expansions in their Air, Land, and Sea expansion pass, and will feature new missions as well as a few other surprises the developers have opted to keep to themselves. The other two expansions have had no details released so far.
As well as the announcement of Sky Fortress, Avalanche announced that weapons and vehicles that were previously only available to those that preordered the game have been released for purchase by the public. These items can be bought on Steam, PlayStation Network, and Xbox Live as of right now.
Lastly, Avalanche mentioned that a patch for Just Cause 3 can be expected later this month and that they will continue providing support for any problems encountered in the future. The patch will include bug fixes as well as performance enhancements, and they encouraged players to contact support if any other problems arose.