Seeing as how we know the traveler and witness are supposed to represent the gardener and the winnower it stands to reason the next arc and major villain would be the vex. ok I know what people are going to say but hear me out the oldest thing in the galaxy were the metal flowers in said garden that became the vex for one and also we know all the vex we ever see except for wyverns are just scientist and researchers with wyverns being the only combat one assuming they have more than one soldier type and how much wyverns are honestly tough as hell it would stand to reason the vex watching from the garden where they can see all of time they may finally find a timeline where they think they can take us out for good especially with the current biggest threat presumably not around to stand in the way also