In the Books of Sorrow, Oryx is shown to show sorrow for the races he'd killed, and even initially tried to broker peace, only to be killed by Savathun. From what I could read, Savathun is the most merciless, she killed tons of races and never showed any sign of remorse. We have also seen, that through the Books of Sorrow, that Oryx is never separate from his sisters. This means that Savathun and the last remaining sister is either in our solar system currently, or is on their way. So perhaps we'll see them in Destiny 2?