That's called arrogance. And while next does mean next, needing their help with something doesn't mean the Legion is next. Maybe they escape and warn people OF the Legion (A much better tie in to a legion expansion) during the next expansion, but I don't see it confirming the Legion, nor your Heroes of the Storm speculation, which has been debunked as you grasping at straws constantly.
EDIT:: To add to that, we may also see more time related stuff that would involve them instead. Interesting how an isle outside the realm of time would appear all of a sudden, and in the very next expansion people who have been missing for ages are going to reappear. Lost in time, perhaps?