Avalanche Software today announced that the development for the first DLC for Just Cause 3 are almost complete, so soon we should know more about this content also included in the Season Pass of the game. But in the meantime the developers anticipated that the title of the DLC is Sky Fortress ( Fortress of heaven ) and that will bring us new missions, features and excitement into the heavens of doctors.

From this I infer that the DLC will introduce some kind of flying fortress that we will have to attack using aircraft in the game: we will update when we know more.