While many think that the past First Order and Reaper invasions were simply there as jokes and a final challange which many found annoying, it also served as a way to test out invasion mechanics for a possible Vong invasion end game event. From what is known it's not coming in 5.0, but it is something that will come eventually. Maybe in 6.0? They are also there for Remake lore reasons (apparently there is deep established Remake lore already, and the Vong have a hand in the many reasons why the galaxy is the way it is in Remake).