Fallen first definitely. There's only one house left that isn't definitively shattered, and thats the Kings. Next would be cabal or hive. The main factors would just be how long it takes them to get to our solar system(assuming we stay in the solar system for destiny 2) since they both were probably called out to by the end of ttk. Vex should totally be last since they're time traveling space robots who keep trying to bring back the black garden and nexus. Wait taken are already dead so they're first.