In addition to the Sword Art Online: Alternative Gun Gale Online anime, Studio 3Hz was also apparently working on another anime series titled Black Fox. However, there's no release date for Black Fox so the series likely isn't premiering until the Summer anime season, at the earliest.
Robotics;Notes and Joker Game director Kazuya Nomura is helming the project. There's still not a ton of detail regarding what the show is about but the trailer below offers some small clues.